The letter, which appeared to be written on official letterhead from Richview Collegiate Institute, warned teachers and students to "avoid eye contact with African American students."
It later read: "Due to their aggressiveness, African American students will be made to pay an extra $1.50 per purchase in the cafeteria."
School principal Sam Miceli told CBC News he learned of the letter from colleagues at other schools, after images of the document were circulated online on social media.
He felt "revulsion" when he learned of the letter's existence, he said.
"It's a very safe nurturing school, school composed of a variety of ethnic backgrounds … So this is an abberation to what we experience at Richview on a daily basis," he said.
The principal told CBC News that if it turns out to be a student who is responsible for the letter, he or she could face serious consequences, including multiple-day suspension.