Two year 7 students at Melbourne Girls College are being investigated by police after dealing marijuana at the top state school.

The school called in police after the girls, understood to be aged 12 or 13, were caught dealing the drug on school grounds.

Principal Karen Money said the two students have not been expelled, with the school choosing to offer them and their parents support.

She said drugs were "not tolerated" at the school and described the incident as "isolated".