Years ago I used one of the clear dome chick bators by gqf. Surprisingly it worked very well. I believe that was the only incubator I ever had that ever got 100% hatch. I probably ran it through 100 or so hatches and never had a bad hatch but of coarse only 100% once.
About 15 years ago I used a little giant Styrofoam incubator. It was terrible. Maybe 5% hatch most of the time(if any at all). It had a fan but would still spike very high on temp. The element looked like one in a stove. Thankfully it only lasted through a few hatches.
About 10 years ago I went with a brower top hatch. It could not do better than 15-20%. It would spike to hot as well.
A few years back I bought a 12 volt electronic Styrofoam gqf hovabator. It performs as well as the sportsmans I have on percentage of hatched eggs. I got it for an emergency brooder if the power goes out. But more times than not it is full of eggs. I am not sure of the price but it is the best cheap incubator I have used. Most likely it would be to high for a tight budget.
Most likely the
http://www.cutlersupply.com/cart/index.php?main_page=product_info&products_id=195 would be the best for a classroom.