Sorry if you think I am being harsh, but maybe if you had to deal with the fallout, when it all goes horribly wrong, then you would understand why I am so against it. I have had schools ask me to take away their older piggies, because they can't have the children upset if they die, so please remove them before they die, but can we have some babies instead? I took away the older piggies but they didn't get any babies from me. I have had schools ask me to take away piggies when they've got health issues, because their budget doesn't extend to huge vet bills, or actually any vet bills. I had a petsitting client ring me to say it was her son's turn to bring home the class guinea pig, and are guinea pigs not like rabbits? Do they not need hay? Apparently the guinea pig had never had hay because some of the children had allergies to it!
And as Wiebke said, be mindful that putting 2 piggies together isn't as easy as it sounds. My 2 didn't get along and are cage neighbors.
One of my biggest worries is the taking home at weekends and in school holidays. Would you be able to take the guinea pigs home with you, as a guinea pig could become ill during a weekend with one of the pupils and it not be noticed, as signs of illness are so subtle? Guinea pigs can go downhill so very quickly, when poorly. Also, what size cage would you be using? The guinea pig (yes, they only had one) that my pet sitting client's son took home, was in a tiny cage, which was not at all suitable. However, I was told that there wasn't room in the classroom for a bigger cage and also, as the guinea pig needed to be moved to different locations for weekend and holiday care, it needed to be small, so that it could be transported easily and not everyone's house was big enough to accommodate a larger cage and it wasn't fair to discriminate against the children who lived in smaller houses.
I think your concerns are all very valid but should perhaps be directed at the local education authority or the school governors rather than at the person looking after the guinea pigs. We intend to offer to foster the guinea pigs we found out that my husband's new school has, during weekends and holidays- and we might well ask advice about them on the forum- but as they are part of an "animal therapy" programme for autistic children who only speak to animals not people, the piggies being in the school environment is a fairly senior decision and a tricky one balancing animal welfare and disabled child development. We would just look after the piggies if needed, because we already have piggies, and lots of piggy food and hay and spare cages and a good vet- but we'd have no input on whether they lived at school or not during the week because essentially they belong to the local education authority. Being put in charge of an animal, or offering to take charge of it, doesnt necessarily mean you have any control over what it does as its day job!
The guinea pig doesn't have 'a day job'. It's a small, defenceless animal, that needs us to speak out for it!