William Docherty, 42, assaulted the teenager in October 2009. Docherty, from Livingston, had denied the charge but was found guilty of indecent assault at Falkirk Sheriff Court.
Sheriff Derek O'Carroll criticised Donaldson's School in Linlithgow, where Docherty worked as a communication support worker, after it emerged he was not suspended until almost four years after the assault took place.
Mary Mulligan, convener of the board of Donaldson's Trust, said: "We took all necessary steps to protect our pupils and informed the police who took it from there."
Sentencing on Docherty has been deferred until next month for background reports.