Students with HIV pose a threat to
the entire school population. They could
infect other students or staff members. Making
HIV positive students reveal their status to the entire school population would
be very awkward and open them to bullying and harassment. Students with HIV should be home schooled or
attend school with other infected students.
Unless the infected student walks around with a syringe full of his blood and injects it into innocent passerby's, there really isn't a risk of the disease being transferred. Normal school contact like bumping shoulders in the hallway or shaking hands with someone with HIV can do you no harm, so there is no risk of an infected person being in school with other students.
Firstly, there are multiple ways that one can be infected with HIV, and simply having it does not imply that they ever engaged in act act that could have been expected to possibly result in the transmission of HIV. Unless they go around trying to inject their blood into other students, or otherwise deliberately spread their bodily fluids into others, it's just not going to affect other students or members of staff. Secondly, it's a viral infection; you can't just expel a student for being sick, regardless of the specifics of their illness.