Because if you are going to put other people at risk, you should have a really good reason.
When we require vaccines for school, we get more children immunized. When we get more children immunized, we prevent diseases—including cancer. When we have a vaccine that works and is safe, we should be making sure that it is given to as many people as possible, and requiring a vaccine for school does just that.
When we require vaccines—including HPV— for school, we not only give our children an education, we give them the best chance at a healthy future.