The Greater Essex District Public School Board has hired security guards to keep an eye on its high schools in Windsor-Essex.
Because of a dispute between teachers and the provincial government, the union representing teachers has told them to stop doing scheduled supervision.
At issue is Bill 115, which gives the government the power to ban strikes and impose a two-year wage freeze.
Helga Bailey is chair of the board of trustees and is concerned about safety.
"So as a contingency plan, we do have security personnel in the schools, to make sure that our students are safe," she said. Their union also said teachers should not attend staff meetings, communicate with parents outside school hours or fill in for absent colleagues.
The province said it's disappointed by the strike action.
Education Minister Laurel Broten says the government will continue to monitor the situation.