Travellers who arrived at a school playing field overnight could be moved on by the end of the day, according to the council.
Over 35 caravans and close to 100 additional vehicles are occupying the site at Durrington High School in Littlehampton Road, Worthing.
A spokesman for West Sussex County Council said: “About 35 caravans first started turning up on the grounds of Durrington High School at about 7.30pm last night.
“We are now conducting checks on the group before serving them with a notice under the Education Act to quit the grounds.
“Sussex Police will be assisting us with the eviction, which we anticipate will be completed by the end of today.”
According to officials, the Education Act helps local authorities to evict travellers swiftly from school property.
Updates to follow.