Bruce Rankin appeared at Hamilton Sheriff Court on his 58th birthday.
He admitted making a sexual remark to the girl and handling her body at the school which cannot be named for legal reasons.
The assault happened between August 2012 and June 2013 when the girl was aged 14 or 15.
Rankin’s not guilty pleas to charges alleging he touched another female pupil, made a sexual remark to another and watched two girls getting changed in a female changing room were accepted.
Rankin, of Whiteshaw House, Strathaven, admitted sexually assaulting two other girls at other locations.
Sheriff Shiona Waldron called for background reports and deferred sentence. Rankin, who will go on the sex offenders’ register, was ordered to return to court next month.