I agree with Su.Ki's points (as I usually do!) but also wanted to mention that there's something not quite right about the first sentence, because it's not the other student who is exposing the children, but some third party, an authority-figure adult presumably. Also, if the exposure is in the present, then the infection would need to be, too. So I might write it as "The school children are exposed to the measles of a student who has it".