During a decade and a half of my regular education the usual way to call students was by their last names. First names can be used in 'chamber' classes: trainings, courses, in-corporative education… When there are 'too many Sashas' several ways exist to resolve the collision: using last names, (full) first names with either patronymics or last names, different forms of the same (first) name — diminutives (which can be many) and a full form. In such cases it’s a matter of a mutual agreement (an explicit one, as a rule) — which form is applied to which person. E. g. a young person can be called
Саша, an older one —
BTW using diminutives requires special conditions. Unlike for English Mike, calling a person
Миша instead of full
Михаил Иванович should be sanctioned either by common social practices/conventions or by that
Михаил; in the last case one should be 'granted' to use
Михаил [Иванович] explicitly, no matter the rest of the world uses but
Миша for him. (Perhaps, in this para, I idealized the real world a little to make explanations short.)
Sasha can be used in school but in higher education it would be a special case for me.
NB. I was a student in the previous century. Modern practices can differ, i. e. I admit that now there can exist institutes/universities where Sasha is pretty OK in general but not that old practices disappear.
As for your question proper, diminutive+patronymic can be used — exceptionally, like exotica, probably with a tint of humour, for such combinations can sound like a joke in Russian. [upd] Also it can be used in a pleading manner to seek someone’s assistance, rarely as well.