The post said: "It went badly wrong. They were intercepted by our ground staff but tried to escape. One of our staff was run over and is now in A&E in hospital".
The school said police were now involved.
"There will be damage to the wing mirror and bonnet," the post said.
"If you know anything about this please contact the school and ask to speak with the principal. If you were involved, do the right thing and take responsibility. Our staff member is recovering but this could have so easily been so much worse."
Police are appealing for any information about the vehicle.