I think that the manga mentioned that there was a "kindergarten, elementary school, junior school, high school, university, and graduate school", in the first 25 levels of the building, which probably could be feasible if there weren't that many students and not much of a variety of classes, or if maybe all the students, even in the college, stayed in their homerooms. The school also has laboratories and halls though, which would make accomodations a bit tight when included. It also makes one wonder what the Professor Tomoe uses the other remaining floors for.
Then again, since the school was a front, it may have just been intended for use as the Death Busters' base primarily. What do you all think?

Pages from manga:

I don't doubt that they could. They state that entry is extremely limited based on talent and excellence, so I'd just assume there is very limited admittance.

In the anime too they said the Mugen Academy housed all the grades including a university, but I don't recall there being any mention of them all being on the first 25 floors. That said, it is all possible given the size of the building and the limited attendance based on how hard it is to enter.

One thing I wonder though is what happened with all the students after their school got destroyed. Do they even remember it existing after all that? Or was it all reduced to a crazy fever dream in their heads like what happens with most people after they fall victim to a Monster of the Day attack?

I'm pretty sure they could-especially if you count all the evil magic at play. Why not have some portals in place to teleport to class, or have some distorted spaces/sub-dimensions where the energy stealing classes could take place? In Crystal, they showed plenty of elevators and escalators too.

OOhhh this is such an interesting question, especially given limited space in Japan. I would agree with Sailor Jupiter and think that it was most likely just very limited students.