Concerned parents have called for tougher security measures at Penistone Grammar School after an intruder was escorted off the premises by staff.
Pupils say the man, in his late teens to early 20s, entered school grounds looking to settle a grievance with a 15-year-old boy.
He was chased off by teachers after approaching students, before police were called last Wednesday.
The school's senior vice principal said at no point was any student threatened, intimidated or hurt.
One parent said: "How is it possible that a man can enter a state-of-the-art school like Penistone grammar?
"We feel that as parents it’s been covered up as, at present, no parents have been informed of the matter at all by the school and only by our children. It seems typical of this school, which is more bothered about its image rather than basic common sense."
Some pupils alleged the man was carrying a knife, although school staff and police said there was no evidence of a weapon.
Another parent said: “Anybody could walk in - where's the security?
"It's scary with what's happening in America and things like that. They're not safeguarding our children and what's upset us more is that we find out from our 15-year-old."
A year 11 pupil said he and his friends saw the man and asked him what he wanted. The intruder is alleged to have given the name of a 15-year-old boy he was looking for, but the boy in question heard the conversation and managed to run away without being seen.
Geof Baddock, senior vice principal at the school, said the intruder climbed over a wall and gained access to the school building through one of the south entrances.
"He was challenged by a member of staff as he did not have a white lanyard worn by post-16 students and was wearing a blue ‘hoodie’ which was not allowed in school," said Mr Baddock.
"He was rude and abusive to the member of staff, who then rang reception for assistance."
The man was escorted off the premises by three members of staff and the school contacted the police. He remained within the vicinity of the school grounds until officers arrived.
A South Yorkshire Police spokesman said despite reports, the man was not armed and was moved on by officers.
Mr Baddock added: "We take our safeguarding responsibilities very seriously and will review our procedures and policies after this incident, as we do on a regular basis."
The school has set up two Twitter accounts to improve communication with parents, following approval from Barnsley Council. One account will be used for years seven to 11, with the other for post-16 pupils.
