Police have been called out after up to 30 caravans and vehicles belonging to a group of travellers moved on to the grounds of a secondary school.
The travellers parked up at 1,800-pupil Hazelwick School in Crawley, West Sussex, after a padlock preventing entry to its grounds was cut.
Education officials said that lessons were running as normal and advised parents to still collect their children at the normal time of 3pm.
Sussex Police said the first vehicles pulled up on a verge close to Hazelwick's entrance just before 11am on Friday before moving on to the playing field.
They are believed to be the same group that set up camp in the town's Manor Royal area and left after being given notice to quit by the landowner, police and the local authority.
A Sussex Police spokesman said: "Police officers are at the scene liaising with West Sussex County Council, as the landowners, and assessing the site."
West Sussex County Council officials said the travellers have been given notice to leave the grounds.
A council spokesman said: "Twenty-two caravans are currently on site having forced entry on to the school field by cutting a padlock.
"The group have been served with notices under the Education Act 2002 and are required to leave the premises immediately.
"The first vehicles pulled up on a verge close to the school entrance just before 11am.
"The group then moved on the school playing field after further travellers arrived.
"They are believed to be the same group that was at Manor Royal who left after being given notice to quit by the landowner.
"We are liaising with police officers at the scene."