I know a teacher back home in South Carolina who loved having lots of class pets, but one year had to get rid of the hamster (someone was allergic), the spider and snake (a student was petrified of them), the turtle because a parent complained they were unsanitary and her bird (forget the reason). Only the fish remained. She was a third grade teacher and she said that pets in the classroom really helped teach responsibility.
I think I'll send her a Webkinz this year for her class. Think about it - anyone who does not get their tag pulled, lose a color, or whatever you do to keep track of discipline in your classroom- has a chance to play for 10 minutes in the "WebKinz" world. They can't spend the money unless it's to buy the food for the day. Then on Friday, as a class, you can purchase another room, new furniture, and other fun stuff. Your pet's happiness and health are monitored, there are some great educational questions in there, the responsibility to feed your pet healthy food choices, and so much more also turns it into a learning reward system. Just make sure that even the kid who can't sit still and gets on your last nerve gets a chance to interact with your webkinz.
And I haven't met a kid yet who's allergic to a computer (that's kept clean), you can't get sick from a computer (that a teacher keeps clean), and who's going to be petrified of a Webkinz? (Just make sure you clean your computer - and there's no excuse!)
I think I'll get my son's teacher one this year for the class.