An old anime (1998-2007) about a guy living with a couple of girls under the same roof
This sounds like Love Hina. Some relevant plot points that matches your description:


The main character wears glasses.
He lives with other girls in a hotel, trying to get into the University of Tokyo. He failed the entrance exams twice and still attempts to get into it.
One of the girls is taking entrance exams to the same university.
He does accidentally see other girls naked at times.
There are no explicit sexual encounter, at least, none that I know of.


I'm not sure about him having a crush on the oldest one. Let us know if this is the correct answer or not.

Source: http://en.wikipedia.org/wiki/Love_Hina