What is usual cause of conjunctivitis in schools?

Pink eye. Viral and allergic. But the most contagious is bacterial, that is why antibiotics are prescribed.
Viral. Conjunctivtis in schools is a moderately contagious viral illness similar to the spread of ordinary colds. It is spread, unlike colds, by direct contact. For instance a child may have it, rub the eye which gets the virus on the hand, open a door leaving the virus on the knob which is opened by another student who can then transfer it to his eye. In this way many students get infected.