I have a bit of experience with classroom pets and have personally owned most of the animals mentioned. I will throw out my thoughts from easiest to hardest for your consideration.
tropical fish - easy and interesting especially if you set up a 10-29gallon aquarium of smaller community fishes such as guppies, platies, mollys, etc. Nothing excites younger children like checking the aquarium to see if the fish have had babies!
Hermit crab, some frogs, some lizards - fairly easy, inexpensive and have the advantage of allowing supervised children to handle the animals. Lizards tend to be a bit fragile and fast so consider slower moving skinks rather than fast and fragile geckos. Oh and NO IGUANAS! they get huge!!!
hamster, rat, mouse - moderate but can make good classroom pets provided they have some general care. allergies and biting are two issues. Suprisingly, rats are probably the easiest and friendliest and by far the most intellegent of the three.
Rabbit, guini pig - Moderate similar to smaller mammals but require more space. I personally think a rabbit is an awesome classroom pet but larger mammals are more prone to trigger allergies. I had rabbits in my classrooms growing up and thought they were great!
Snake, aquatic turtle, land turtle - Difficult but can be fascinating pets for older children. Of the three, snakes are a bit easier and would probably get a moderate rating if the correct species is selected. However, they do need special diets and lighting with temperature gradients. Corn snakes probably make the best snakes for the classroom as they don't get too large and can be extremely tame if raised in daily contact with humans.
Birds, dogs, cats - A generation ago, they were common pets in the classroom but are not well suited since so many children have allergies to them today.
Good luck on your research and I am a HUGE believer we need animals in the classroom!
Matt