An assailant, in two attempts within 10 minutes, cut the girl's hijab using scissors while she was walking to school with her brother in the city's Scarborough area, a Toronto police spokeswoman said.
The hijab is a head covering worn by some Muslim women and girls. It covers the hair but not the face.
"I felt confused, scared, terrified," Khawlah Noman, who is in Grade 6, told reporters at her school on Friday.
"I screamed. The man just ran away. We followed this crowd of people to be safe. He came again. He continued cutting my hijab again."
The Toronto District School Board said it was "shocked" to hear about the assault which Ontario Premier Kathleen Wynne called a "cowardly act of hatred" that did not represent the province. Police did not have anyone in custody.
The attack comes as Canada approaches the first anniversary of a deadly shooting in a Quebec City mosque that killed six people at prayer.
A French-Canadian university student has been charged as the sole suspect.
Last month, a Quebec judge suspended a law banning people from wearing niqabs and other face coverings while giving or receiving public services.
Researchers have documented an increase in far-right extremist activity in Canada, much of it targeting Muslims.
A survey conducted last year by Ontario's Human Rights Commission found that more people reported harbouring "very negative" feelings about Muslims than about any other group.
The National Council of Canadian Muslims has called on the federal government to declare Jan 29, the day of the mosque shooting, a day of remembrance and action on Islamophobia.
The government has not said whether it will do so.
