A GANG of ten youths threw stones at staff at Oasis Academy Wintringham, Grimsby.
Senior managers were forced to lock the school gates at the Scartho Road entrance and open an alternative emergency exit for some of the 900 pupils to leave in safety.
Humberside Police attended the disturbance at the school gates at about 3.20pm yesterday.
No arrests were made as the gang, understood to come from a rival school, dispersed.
Principal at the Oasis Academy Jane Bowman said: "It was students from outside who were at the gate. Most of them were youths in hoodies, but a few looked older.
"The group had turned up and were causing a nuisance and disturbance. When we went to confront them, they dispersed in all directions but later reappeared at the gate.
"Why they were there in the first place, we don't know, but they were, and we don't want to see that sort of behaviour at the end of the school day.
"We heard rumours that they had come to the school to meet with one or two pupils from here. But our over-riding concern was the safety of all our pupils.
"After they ran off they came back again so we called the police. I ordered that the gate should be kept locked until we knew it was all clear.
"The paramount objective was to get all the children home safely. We opened our emergency exit out at Park House and arranged for most of the pupils to leave by that exit and the police asked us to use the Weelsby Avenue gate for the children to leave."
Miss Bowman said she tried to engage with the group of 10 youths, along with other staff.
She said a number of them had climbed on top of the cemetery gates.
The principal said: "They were making a real nuisance of themselves."
A Humberside Police spokeswoman confirmed officers were called to the scene.
She said: "Enquiries are on going to establish the nature of the incident.
"No one was injured and no arrests have been made."
The violence came as pupils prepared for their summer fair which went ahead unaffected by the drama at the school gates.
Have you seen our new website? More news on grimsbytelegraph.co.uk Bookmark grimsbytelegraph.co.uk/news for the best breaking news and exclusive local content. Follow us on Facebook and Twitter
