Travellers moved into Angmering School playing field the night before Sports Day.
A group of around 20 caravans moved into the school field in Station Road at just before 9pm last night, police confirmed.
Most moved on later in the evening, a police spokesman said, but about nine remained there this morning (July 9).
The Sussex Police spokesman said they 'intended to move on'. He said they left shortly before 7.30am.
A spokesman for the school confirmed Sports Day still went ahead as normal.
Sussex Police said it is committed to promoting equality and respect for diversity across the gypsie and traveller communities in the way it serves Sussex in delivering policing and also as an employer.
Superintendent Howard Hodges, Equality Champion for Gypsy and Traveller Equality work in Sussex Police, said: “As the gypsy and traveller equality champion for Sussex Police I am determined that the community is treated the same as any other community that we serve, so that the service we provide is accessible, responsive and visible and treats the gypsy and traveller community with fairness, respect and dignity.”