The Toronto District School Board says a student at York Memorial Collegiate Institute has tested positive for COVID-19.
However, a note from the school principal suggests the risk is low.
In a letter to parents on Thursday, Principal Donna Drummond wrote, "please know we have no reason to believe there is any cause for concern for students and their families as this student was very briefly at the school on Monday, was not at school today and will not return until cleared by TPH.
The first day of school for most high school students was Thursday but students in Special Education Congregated Sites and Intensive Support Programs began on Tuesday.
The school board says the student who tested positive was only briefly in the school before the start of full-time classes.
"As a precaution, an enhanced cleaning was conducted in the impacted areas but no further steps have been advised by TPH at this time," Drummond wrote.
Because the case was found before the first day of school, she says parents will not be getting a letter from Toronto Public Health.
"Having said that, we just wanted to let you know about this particular case as it must be reported to the Ministry of Education and Toronto Public Health," Drummond wrote.