The two hottest resolutions for classroom projectors are XGA (1024x768) and WXGA (1280x800). The big difference between these is that XGA is a 4:3 aspect ratio, and WXGA is 16:10, or widescreen. These two resolutions are popular for classrooms because they are very low in price but they put up very large, bright pictures.
For schools on extremely low budgets, the older SVGA (800x600) resolution is still available as well. Like XGA, it is a 4:3 aspect ratio, but it is lower in price than XGA.
Since XGA, WXGA, and SVGA projectors are very different in cost, resolution and aspect ratio, we have three separate Top 10 lists:
Top 10 XGA Classroom Projectors
Top 10 WXGA Classroom Projectors
Top 10 SVGA Classroom Projectors