Some pets are Ok as permanent members of the class room, but it does largely depend on the class and the individual animal. The main issue is who cares for the pet on weekends, holidays (like spring break) and during the summer. The pet also needs to get the care and respect it deserves during class time.
I think that the teacher needs to be 100% responsible for the pet. They should be the ones caring for the pet during school breaks and making sure the pet is comfortalbe during the day. Some of the daily care like feeding and cleaning can be done by a small group of students. The group should be small so that the animal is not overfed and you know that the students that do it care. The teacher still needs to be the one who supervises the students and provides all things the pet needs.
Ideally, the pet (if at all possible) should go home each night with the teacher. This is easier for a pet like a rabbit or guinea pig.
I would avoid having pets in the classroom for really young kids, basically grade 3 and under. These kids are more easily distracted and could be too excited and energetic for the pet. Older elementary students and those in junior high or high school would be better.
Having the teacher bring in a pet for a week or so a couple times a year would probably be better than having a pet there all the time. The students would get more out of having the pet there and it could be incorporated in the to lesson plan. Some class actives could use the pet in them.
Although I never was in a class with a pet (other than a fish tank in grade 7), I have been involved with a school that does. This school is an elementary (K-6). They mostly have birds and the birds are not in a classroom. There is a main hallway that has some natural light and 2 bird cages. There is a bench and some bird books so the students can sit and watch the birds. There are also 2 cages in the kindergarten room. I believe there is a group of students who care for the birds. I am not sure what happens to them during breaks, but I think they are taken in to homes for the break and some back to the school when school starts again.
My brother apparently had a rabbit in one of his classes in junior high. I think the rabbit was free to roam around the class room. I didn't go to that school, so I don't have any details.
Reptiles and amphibians do not make good class pets. They have very specialized care and can be expensive to keep. There are also some diseases associated with them that would not be good for children.
Small mammals like mice may not be good, but if only a few people handle them, it could work. Some are nocturnal, so should be avoided.
Large animals will not fit very well in a class room. Any class pet should have a cage and some cages can take up a lot of room.
Noisy pets like birds are probably not the best.
I have heard that Guinea Pigs can make good class pets. You would want a large cage and to take precautions with the students.
Some rescues will bring a few adoptable pets to classes so the students can learn about them. It could be a way to have some pets around without having to care for them. You could contact a breeder and see if they are willing to bring a few of their animals for a couple hours to teach the kids about that species.
Many children have allergies to pets. Some don't even know if they are allergic to the pet you are wanting to bring in. You would need to get the principals and parents OK to do it and be prepared if a child does have allergies.
