I'm going to second the recommendation to research your local animal neglect statutes before you escalate the issue. Different jurisdictions can have wildly different regulations on animal welfare, and you don't want to go citing stricter regulations than actually apply in your town/county/etc..
It may also help to cite a learning/role-model angle as well. Point out that it's the teacher's responsibility to set a good example of responsible animal husbandry for his students, and that this sorry collection of pets is doing nothing more than teaching your fellow classmates that it's okay to neglect something that they're taken responsibility for. That might induce the admins at your school to pay attention more than whatever technical proof you can muster that the animals are being mistreated.
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