Constable Bruce McLean said people needed to realise the school was "effectively a closed yard" and, as such, anyone found to be in the grounds or buildings without authorisation was liable for prosecution.
"It may seem relatively innocuous, but it is not a public space," he said.
Recently, police had dealt with people being at the former school without any authorisation and using the grounds for their own purposes.
A "reasonable amount of construction" had been carried out by some skateboarders, who had built a 2m-3m high concrete ramp on the school grounds, he said.
"It's a good time to remind people you can't go there; you can't hang out there, even though it is not being used," Const McLean said.