Digital cameras are an incredible enhancement to the classroom. Images of lab setups, results, microscope images, and the people in the group can all be recorded and included in a report by the student. No processing time or expense are involved over what is already in the modern classroom.
However, microscope adapters can sometimes be non existent or very expensive. A simple adapter can be made from a discarded Kodak film can [the solid black ones, not the clear ones]. Often times camera stores will have drawers full of them and are happy to part with a few.
Remove and discard the top. Cut off the bottom with a knife [teacher does this] or small saw.
(Cross sectional views)
prepared slide from Ward's Scientific, 92 W 8331.
10X objective and 10X eyepiece
student grade microscope
Contrast enhanced in computer afterwards
As a teacher you might want to have a "good" setup for your microscope. The adapter is not cheap, around $300, but it is professional and will allow images as good as your optics can handle and it is capable of leaving the camera on the microscope. Most digital cameras will allow continuous video feed to a monitor or LCD projector allowing you to demo things to the class as a whole and record images of anything interesting for later review.See Suppliers.
Regarding megapixels (MP), generally the more megapixels the higher the cost. No point in a student camera being above 2 MP, even 1.3 MP would work well. Their scopes just aren't capable of higher resolution. For the teacher scope, 3 MP is more than enough.