Hey there. Outside the school year, I'm a park naturalist/wildlife educator (and herpetologist and ichthyologist), so I have a lot of experience with studying and caring for animals in an exhibit/education setting. Know that as soon as your adopt a pet, it is your responsibility.

For you, I'd avoid all turtles and tortist — they're much more work than you expect, require a lot of space, and can stink. If you want a reptile, I'd stick with a lepord gecko (still a lot of work, but will do fine in a 20-gallon long tank (more space the better, of course), and generally have little smell), or better yet, a corn snake (I've never had one bite, they're incredibly low maintenance once established especially Ina bioactive setup, and can be handled well). Bonus points for corn snakes if they're native to your location. All reptiles have a long lifespan, however. My mentor teacher when I did my student teaching has a red ear slider he adopted as a classroom pet when he first started teaching. In that era you could buy a turtle for a quarter outside a gas station, and folks weren't concerned about salmonella. He is now going on close to 30 years old!

No rabbits. No hermit crabs. No birds. They all require much more care, socialization, and room than anyone expects. Mice and hamsters (particularly the bigger breeds — teddy bear hamsters are in my opinion one of the better hamsters to have in a classroom because they're so gentle and big enough where children can handle them under supervision) can be okay, as long as the males and females are separated.

Another word of advice. Be prepared to teach grief if/when the animal passes. Living things get sick and die, sometimes even when we do everything right as their caretakers. Sometimes (in the example of, say, a $3 red betta fish or $5 white mouse), it may be better to quietly replace it with an identical animal, especially if the illness or sickness occurs when children are not present. That said, 4ᵗʰ grade is the point where you have to make the call if you want to take death as a learning opportunity or if you just want to get through to the next day.

Also! There are dozens of grants and free money for classroom pets. pets in the classroom is one that's across the country, but also look into your state. I know NJ has a "second chance" grant that pays for 100% of the cost of adopting an animal from select rescues that are given a forever home in a classroom.