I remember this one elementary school class I was in had a small fish tank with a betta in it. I don't remember its name, but what I do remember was that it was probably named after a Spongebob character.
I never had any classes with pets until my sophomore year. My history teacher at the time kept a snake, and she let me hold it when I needed a little sensory stimulation. The snake doesn't mind, and it hasn't bitten me before.
More recently, in my science class, there was a bigger fish tank in the back and it held three fish.
I would also like to mention that I know of this one teacher who brought his pet rabbit to school. I saw him walking and holding the rabbit's cage when I was waiting to be picked up by my parents.