"We would like to hear from anyone who has information which could help us identify this offender or may have been in the area near the school grounds."
The man was also seen in the Somme Cres, Hillcrest, area on July 26 and 27, 2017, and possibly had a bicycle with him.
He is described as of medium to large build, aged in his 20s, possibly with a beard or some facial hair.
At the time of the incident, he was wearing dark-coloured clothing and a dark-coloured hat or cap.
Police would like to hear from anyone who has seen a man of similar description in area acting unusual or appearing out of place.
There was a dedicated team of detectives working on the investigation and it was being treated as a priority, police said.
Any information could be shared with Hamilton CIB on (07) 858 6200, or provided anonymously to Crimestoppers on 0800 555 111.