Recalling his high school years, in his An Autobiographical Study, of 1925, Freud wrote, At the Gymnasium [Grammar School] I was at the top of my class for seven years; I enjoyed special privileges there, and had scarcely ever to be examined in class.*
The claim is odd. Whether Freud was a top student or not, one doesn't escape being ever examined in class. After all, how else would the teachers determine who deserved to be at the top?
According to Freud's biographer, Ernest Jones, For the last six of the eight years he stood at the head of his class. ** This contradicts Freud's above claim that he was at the top of his class for seven (out of eight years).
Was Freud intentionally lying, which is not impossible, or was he simply misremembering? After all, when writing his confession, he was already 69.
How do we know about Freud's high school marks at all? As Jones revealed, Dr. Bernfeld has retrieved from the school records Freud's place in every semester. Notably, as Jones pointed out, When his sister says he was head of his class for eight years, and matriculated at eighteen, she makes two slips of memory. (Just like her famous brother, his sister lied about Freud's high school achievements. And Freud graduated at 17.)
* SE 30, p. 8.
** Jones, Sigmund Freud: Life and Work, (1953, p. 20).

So, what kind of student was Freud? In his letter of June 16, 1873, to his friend, Emil Fluss, informing him about his own graduation exam, Freud wrote, You never would listen when I accused myself of laziness, but I know better.  
Oddly for allegedly a top student, Freud referred to the graduation exams as, my "martyr", adding that, Whether the [written] Matura [graduation exam] was easy or difficult, I cannot judge. Apparently, it wasn't easy for him.
Freud wasn’t entirely happy with his performance. That I sometimes had good and sometimes bad luck goes without saying, he wrote, for the five papers I got excellent, good, good, good, fair (ausgezeichnet, lobenswert, lobenswert, lobenswert, befriedigend). Very annoying.
Annoying indeed, since Freud's grades were rather average, with only one excellent. Freud needed more "luck" to perform well.

Academic fraud in high school graduation

In Latin we were given a passage from Virgil which I had read by chance on my own account some time ago; this induced me to do the paper in half the allotted time and thus to forfeit an excellent.
So someone eise got the excellent, I myself coming second with good (lobenswert).
Only a good mark, even though he had read this very passage by chance ... some time ago!
What a wonderful coincidence.
The German-Latin translation ...  failed miserably. Result: fair (befriedigend). Two others managed to get good, the alleged top boy coming far behind.
The Greek paper ... came off better: [It was] the only good (lobenswert).
Freud explained: This passage I had also read on my own account. Another wonderful coincidence.
This is the second time that Freud knew in advance what he would be examined upon.
Although Freud was hopeless at maths, The math paper, which we approached in fear and trembling, turned out to be a great success: I have written good (lobenswert) because I am not yet positive what I was given.
Yet another coincidence?
Could Freud have known the maths questions in advance, which would explain the good mark?
Finally, my German paper was stamped with an excellent (ausgezeichnet). It was a most ethical subject on “Considerations involved in the Choice of a Profession”.
This was the only subject in which Freud received the highest mark. This is how he achieved this result:

Repeated more or less

I repeated more or less what I wrote to you a couple of weeks ago, although you failed to confirm it with an excellent.*.
Lo and behold. Yet another, the third, "coincidental" knowledge of the examination subject in advance
No wonder, Freud managed to pass the graduation exams with acceptable results. The letter opaquely hints about Freud cheating on the exam, with Freud referring to the inexplicable strokes of luck that people tell each other “among colleagues".**
The hint is, no doubt, related to Freud having inside information about the upcoming exams.
Freud ended his letter stating simply, This, dear friend, was my written Matura.
Unfortunately for his future patients, Freud didn't fail.
* Freud, Sigmund, Letters of Sigmund Freud, (1960, pp. 3-4).
** Freud, Sigmund, Briefe, 1873-1939, (1980, p. 5). [Own translation]

The above-mentioned version of the letter to Fluss, edited by Freud's son, Ernst - thus, no doubt, bowdlerised - was published in 1960. (An earlier version, of the same letter, appeared in Freud's biography by Ernest Jones in 1953.) 
Even though censored, this letter reveals Freud as a liar and cheat, always looking for "shortcuts" to promote his career This pattern would continue throughout his university education, and beyond. (As Freud explained, he was suffering from "compulsion to repeat".)
Out of the five subjects of the written graduation exam, Freud knew, in advance, of at least three of the tasks. There's no doubt that Freud had had access to inside information, about the questions on the upcoming examination, thus, allowing him to prepare himself in advance. Notably, even though he knew the exam's questions in advance, Freud's rather average marks show that he wasn't the top student that he claimed he was.

Professor Hammerschlag Freud's sugar daddy

There was one teacher, at Freud's school, to whom Freud closely attached; his Jewish religion teacher, Samuel Hammerschlag. No doubt, it was he who provided Freud with the upcoming exam's questions.
Not only that. Professor Hammerschlag was one of Freud's several sugar daddies. As Freud recounted in his letter to his fiancé of January 10, 1884, on this special occasion, The old professor ... informed me that a rich man had given him a sum of money ... and he was herewith handing it to me. If it sounds like a fairy tale, then it most certainly is. No doubt, Freud was paid for his services, but for what kind of services?
As he informed Martha, It is not the first time the old man has helped me in this way; during my university years he often, and unasked, helped me out of a difficult situation.
In other words, money for nothing, isn't that nice? As Freud recounted, this was because of, the deep-seated sympathy which has existed between myself and the dear old Jewish teacher ever since my school days.*
Apparently, Freud was a kept boy since his high school years.
* Freud, Sigmund, The Letters of Sigmund Freud, (1960, p. 86-87).

Besides the letter to Silberstein with the final marks, there's also a graduation protocol, on display at the Freud gymnasium, summarised below by the former principal of the Freud Gymnasium in Vienna, who wrote:
I can provide Freud's marks in the "Matura" ("Maturitäts-Prüfung" in 1873) examination: "Matura" (in Germany: "Abitur") has traditionally been, and still is, a written and oral examination at the end of grammar school, at the age of eighteen, qualifying for university entrance.

Religionslehre: vorzüglich
[Religion: excellent]
Lateinische Sprache: vorzüglich
[Latin: excellent]
Griechische Sprache: vorzüglich
[Greek: excellent]
Deutsche Sprache: ausgezeichnet (excellent, perhaps still better than "vorzüglich", as German was Freud's favourite subject)
[German: excellent+]
Geschichte und Geographie: vorzüglich
[History and Geography: excellent]
Physik: vorzüglich
[Physics: excellent]
Allgemeine Naturkunde: lobenswert
[Natural History: laudable]
Mathematik: vorzüglich
[Mathematics: excellent]
Propädeutik: vorzüglich
[Propedautics: excellent]
The principal added the following information:
Reife zur Universität: mit Auszeichnung (qualified for university admission: with honours).
There is a facsimile of the written record of Freud's Matura, which is now at Österreichisches Staatsarchiv. (Austrian State Archive).

A simple comparison of the marks from the written exam with the graduation reports shows that Freud's marks in the final report "improved" significantly. This is odd since, when it comes to combining marks from both written and oral exams, it is always the marks from the written exam that carry more weight.
  Letter to Fluss of June 16, 1873
Graduation report: Written exam:
Latin: Excellent Good/fair
(vorzüglich) (lobenswert/
befriedigend)
Greek: Excellent (vorzüglich) Good (lobenswert)
Maths: Excellent (vorzüglich) Good? (lobenswert)

Moreover, there's a discrepancy between the final marks in the graduation report and Freud's in the letter to Silberstein of  July 10, 1873.

                                            Letter to
                                          Silberstein of  July 10, 1873
Graduation report:                                 Matriculation certificate?
HIstory & Geography: Excellent Commendation
Geschichte und Geographie    (vorzüglich) (laudable) 
Natural History Commendation
Allgemeine Naturkunde       (laudable) N/A

Just like the letter to Silberstein, also the graduation report contains one commendation, although in a different subject; not in the Geschichte und Geographie (History & Geography) but in Allgemeine Naturkunde (Natural History).
Whoever was forging the report attached the commendation mark to the wrong subject!
Last but not least. Bizarrely, for a sex-crazed person, Freud's, Moral behavior: [was] exemplary!

As a "just" reward for his "achievements", Freud's former school, the Leopoldstadt Real and Ober-Gymnasium, was renamed Sigmund Freud High School in 1989.
Subsequently, on April 15, 2005 - a great moment in the history of the Austrian education - Sigmund Freud's forged Matura protocol was handed over to the Austrian State Archives for safekeeping.
NB. Freud was a founder of the now defunct pseudo-science of psychoanalysis. Even so, Freud's 150th birthday was solemnly celebrated in 2006 in Austria.