I have a guinea pig in my classroom and the students love it!! As to allergies, they do cause allergies. I thought I wasn't allergic to anything but when I cleaned the cage for the first time I had an allergic reaction. I can handle the guinea pig no problem and my student with asthma does not have any problems either, but when I clean the cage I wear a mask so that I don't react.
Guinea pigs get lonely as they are social creatures so it is probably good if you get 2 if you are not taking it home on the weekend. I take mine in a little cage home everyweekend. I have a big cage at school and at home that he stays in. But he is sort of more my pet than the students I guess. You definately have to consider what to do with them over the summer if you are away. My husband is a highschool biology teacher and he has lots of pets so we bring them all home in the summer if we are away and hire a student to feed them, a trustworthy one!
Also from what I have read you are not supposed to use cedar chips in their cage.
There are also some good websites that you can check out as to what types of fresh food to give them.
I am sure that I sound like I go over board but I have always been the sucker for animals and believe that if you are going to have a pet you should get lots of information on it and do the best you can to care for it.
Good luck with your decision!!!