There has been a police presence at Blandford School in Dorset, near Bournemouth, since last night after several caravans gathered on the school field.
Around 10 caravans are currently parked in the field next to the school and Blandford Leisure Centre, Bournemouth Echo reports.
The leisure centre warned of the "emergency situation" last night, which caused disruptions to customers and prevented vehicles from accessing the property. Normal access resumed at 6.30am on Friday.
Its statement read: "We apologise for the disruption to our leisure centre customers since late afternoon, due to an emergency situation on the school site.
"There will only be pedestrian access to the leisure centre for the remainder of the evening. Normal access for vehicles will be in place from opening time tomorrow morning at 6.30am."
Blandford School headteacher Sally Wilson issued a letter to parents warning that one of the entrances will be closed and all buses will have to park further away.
She said: "Some of you may be aware there was an incident this evening on our site. As a result the Leisure Centre entrance is closed tomorrow, police will remain in attendance overnight and tomorrow. There will be no access to The Blandford School for pedestrians via the gates next to Block One.
"All buses will be instructed to come to the Milldown entrance where students will get off and then walk down to the main reception via our internal roadway. All students who walk to school are asked to enter the school via the Milldown entrance. There will be staff on duty at the Milldown entrance and along Milldown Road.
"There will be no access for car drop off, if you normally drive your child to school, please drop them off away from the site so we leave Milldown Road as free as possible. Lessons will be as normal. Tomorrow's Business Studies exam will continue as normal.
"There will be a further update regarding procedures for the end of the day later tomorrow morning, parents are asked to keep an eye on our website and social media feed. Thank you very much for your support."
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