Actually Erwartungen does not urgently require prepositions, and the most idiomatic use may be without preposition. (You expect something and either it happens or it does not.)
Die Erwartungen haben sich nicht erfüllt.
Now there are some plausible contexts:
- from whom you expect something as in
Especially if you express that expectation, you address them to somebody and an actually specifies the target of that address. (Such a stated expectation borders to a Forderung [demand], for which an is by far the most common preposition.)
Example:
zu hohe Erwartungen an die Genomforschung
Better would be a phrase using the verb:
Von der Genomforschung hätte man sich mehr erwartet.
- in which respect you expect something
Bis Ende des dritten Quartals 2000 habe Intershop alle Erwartungen hinsichtlich (in respect to) Umsatzsteigerung und Ergebnis erreicht.
Die Erwartungen bezüglich der Anzahl errungener Medaillen haben sich nicht erfüllt.
- the measurable degree of what you expect
This reflects your example, e. g. one expects a good grade but just receives a mediocre one. The best translation I can come up with is
Die hoch gesteckten Erwartungen haben sich nicht erfüllt.
I don't see, how expectations can address a grade, which is just a passive result, and therefore I see no justification for an here.