PLEVA is not contagious. The name refers to a rash which looks a bit like chicekpox, but is not viral or infectious in any way. It is human nature for people to treat almost every skin disease as contagious, and of course kids don't know how to suppress their instincts tactfully. (Many adults are not mjuch better.)
Most PLEVA cases go away before too long, but the school should get involved in minimizing the embarrassment and isolation your brother may feel to the extent that he has to expose affected skin to the gaze of other kids. Get his dermatologist involved in this too, at least to the extent of writing a note.
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