Sara Nicole Girard, 17, from Stouffville, Ont., was thrown from the vehicle in the crash and pronounced dead at the scene.
Two other female teens — identified as Allison Neville, 17, and Meghan Timewell, 18 — were critically injured in the accident and airlifted to a Toronto hospital. All were from the Stouffville area.
The OPP say the three young women were travelling south on Highway 11 in a brown 2003 Pontiac Montana van.
The van left the highway, lost control and rolled over near the village of Burk's Falls, at about 5:30 p.m. Monday.
There were no other vehicles involved in the accident, according to police.
Girard's classmates said they were in disbelief by the news.
"I started crying because I went to school with her," Eric Valleau said.
"You know, she's really, really young, and on top of that, she just graduated, right?"
The southbound lanes of Highway 11 remained closed until the early hours of Tuesday morning as police investigated the crash.
Girard's high school, St. Brother André in Markham, is planning to hold a memorial for the victims.