Southwood Secondary School in Cambridge is being closed for the rest of the week to students and staff for safety reasons after a fire broke out Tuesday behind a partition wall in a second-floor boy's washroom.
Weekend and evening activities at the school are also cancelled until further notice.
The Office of the Ontario Fire Marshall has completed its investigation, but school officials say a police investigation is still underway.
No-one was hurt during Tuesday's fire. The school has 852 students in attendance.
The school board is asking students parents and teachers to monitor the school's website or local media in order to find out when the school will be re-opened.