Dozens of pupils took part in an angry demonstration following the suspension of a teacher this week.
About 70 students from Halifax High School in West Yorkshire took part in the protest on Monday morning after Akram Ali, a maths teacher, had been disciplined.
The school would not disclose the details of the suspension, saying it was a "private school matter", but it did admit that a protest took place after morning break.
It is understood the demonstration had started peacefully. But when more pupils joined in, the police were called to "guide" them back to school.
Local reports claimed the pupils instigated a riot and were guilty of throwing stones at passing vehicles and lobbing cans at shop fronts.
Christian Markham, the headteacher, denied there had been a riot, claiming the pupils were "simply copying" a similar demonstration at a nearby school (??).
Mr Markham said: "There was never any kind of riot here. The kids took part in a peaceful protest that became less peaceful, but I have heard no reports of stones or cans being thrown.
"Half the kids there didn't even know what they were protesting about. As far as we're concerned, the matter has been dealt with. All the kids were attending their lessons on Tuesday and Wednesday."
West Yorkshire police confirmed that officers were called to "investigate reports of a disturbance and criminal damage".
A spokeswoman said: "They monitored the area and provided a presence for the rest of the day. They remained to ensure pupils left school peacefully."