Amscope is a quality and budget conscious option for both dissecting and compound microscopes. You can find decent models in the $200-$300 range. I'm a botanist and use Amscope's at home now that I no longer have easy access to the lab scopes, and I've been very impressed with them.
For the compound microscope (the one you use with slides), make sure it has 4x,10x, and 25x power in the objectives and 10x in the eyepiece. Many also have a 100x "oil immersion" objective, but this is rarely needed at a middle or even high school level. I opted for the Amscope trinocular head, in part because I could peak in when DS was using it to make sure it was on what I wanted him to see, and also so I could hook it up to a computer camera (was included with the scope). Also, always get one with it's own light source. There is no good reason to use cheap and old fashioned varieties that depend on a mirror to reflect a light source.
Dissecting microscopes don't need slides. You can stick nearly anything small beneath them to view easily. I personally use them for dissecting flowers and plants for identification, but they also work great for petri dishes (ex. bacteria growth projects) or insects. If you go this route, choose a microscope with both a back light and an above light, as some things are better viewed when lit from underneath while others can be seen better when lit from above. A 2x-4x objective range combined with a 10x eyepiece should be plenty. Amscope has some dual powered models that can run off electricity or a battery, which is neat if you want to take a scope into the field or on a camping trip, but it isn't a required feature.