The first decisions to make are, "what type" and "how many". At Project MICRO's level (middle school), bigger, more powerful, and more expensive isn't necessarily better. A classroom set should include simple, rugged scopes (built of metal, NOT plastic) with decent optical and mechanical performance, with enough of two or three types for small group use. A good choice would be five or more monocular "dissecting" scopes at $70-100 each, some 3-objective monocular compound scopes with LED illuminators, $150 -200 each, and 5-10 30x hand-held scopes (the flashlight sort), $15 each. Total, about $1000. That level of cost is usually relatively easy to get from local donors. If the available funding won't support that, avoid the temptation to purchase "toy" microscopes with plastic bodies and lenses; supply the class with the "flashlight" scopes, and start looking for funding for the rest.
I30x "flashlight" style microscopes are a good choice for low cost school microscopy; they're widely available in the $15 price range from school suppliers and "science" stores.
Monocular Inspection/Dissection Scopes
Ten or more years ago these were easy to find for around $70, but that's no longer so; you may be forced to buy a binocular. See Project MICRO's "Buying microscopes" advice.
3-Objective Monocular Compound Scopes
There is a large selection in this category; follow the advice given in Microscopic Explorations and on this website, and shop for a good combination of price and quality. A microscope specialty store is most likely to offer a good guarantee and in-house service.
Single-Objective Compound Scopes With Sliding-Tube Focus
Although this design has been purchased extensively by schools because of its ruggedness and simplicity, focusing is often difficult for a child. Project MICRO feels that the 3 types described above are a better choice. Examples are:
Project MICRO welcomes your comments. Contact the co-chair, Caroline Schooley if you want further information.