Accused K Raju misbehaved with the girl, a UKG student, on the school's premises, following which the victim informed the matter to her parents.
Her parents, along with some residents gathered in front of the school and beat up the watchman before they handed him over to police, Musheerabad Police Station Inspector B Mohan Kumar said.
A case was registered against the watchman, who was charged for outraging modesty and was subsequently arrested, the Inspector said.