Police say she crossed the road to get away from him, but then he followed her.
She then boarded a bus from a stop outside the Countdown on Riccarton Rd to get away from him, Stills said.
She reported the incident to police and her school on Tuesday night.
The man has been described as a European, in his late 20s, with short blondish brown hair, clean-shaven and about 188cm tall.
Stills said the girl acted appropriately by crossing the road and then getting into a crowd of people.
"We are looking for anyone who has seen a man that fits this description to come forward," he said.
Recently, a man was trespassed from Villa Maria after he was seen observing students coming and going from the school in a suspicious manner.
"We are confident it is not the same person," Stills said.
It comes after another 15-year-old girl was followed by a suspicious man in Rolleston on June 12.
The girl told police she was followed for about 1km after noticing the man at the intersection of Brookside Rd and Rolleston Drive about 2.30pm.
The man was described as European, in his 40s and about 185cm tall. He was wearing camouflage trousers and a dark grey T-shirt and carrying a camo backpack.
She crossed the road several times but the man continued to follow her to the intersection of Burnham School Rd.
The girl then asked for help from a member of the public, who drove her to her destination.
The man fled in the opposite direction.
Information about either incident can be provided by phoning 363 7400.
Information can also be given anonymously through Crimestoppers on 0800 555 111.
