University of Toronto Summer Math Academy
The University of Toronto's Summer Math Academy is the leading selective mathematics summer program for high school students in Canada, where students undertake intensive study in a higher-level mathematics topic for 1-2 weeks.
Past topics include, graph theory, knot theory, classification in mathematics, and prime number theory. Seminars and colloquia are led by graduate students and postdoctoral fellow affiliated with the University of Toronto Department of Mathematics.
Admission is selective, with around 25 students from across Canada selected to attend. Selection is done through solutions to a problem set with five questions, and a short 250-word personal statement detailing one's interest in mathematics.
More information is available at their website https://mathplus.math.utoronto.ca/home/ma.