Who can medically assess an athlete and clear them to return?

Only physicians and nurse practitioners. Ontario's information for health care providers states they are specified as the only health care providers who can medically assess an athlete and confirm medical clearance. Rowan's Law does not specify the form the confirmation takes, so a sport organization may ask for a medical clearance note. The athlete provides the confirmation, or a parent or guardian does if the athlete is under 18.

Sources: https://www.ontario.ca/page/rowans-law-information-health-care-providers and https://www.ontario.ca/page/rowans-law-concussion-safety