It all depends on the usage in the classrooms. Some may be interactive models, and some may be used just for lecturers/presentations.

As a golden rule, you want to know the aspect ratio of the screen you're working with, the size of the screen in diagonal. Use width/height as well to determine the square footage.

In a normally lit room, you want no less than 80 advertised lumens per square foot of screen space.

You want a short throw or more likely an ultra short throw (UST) projector when interactive white boards will be in use.

I would always pick LCD over DLP in mixed use environments when possible. Epson is also one of those brands I consider a default.