There is actually a medical condition (name escapes me at the moment) in which the unfortunate person exudes an unpleasant odor although I seem to remember that these people are usually aware of it, and seek medical help. I don't believe for a moment that anybody would deliberately do anything to make him- or herself smell awful, but whatever the cause is, I think the best way to handle it would be for you and one or two others to see the teacher quietly and explain the problem, and leave the teacher to probably communicate with the parents.. But if you do go to the teacher with this problem, whatever you do, don't be a'gigglin and a'laughin, or the teacher will get the idea that you are just pulling a stunt, and not take you seriously. If that teacher is going to stick her neck out and talk to that girl's parents, she is going to have to feel confident that the complaint is legit.