This is a link to the Gauss, Pascal, Cayley, Fermat, Euclid and Canadian Open Mathematics Contests. You will find old contests with worked out solutions. For individual contests click on the one you want.
PASCAL (grade 9) CAYLEY (grade 10) FERMAT (grade 11) EUCLID (up to and inclulding Grade 12), Euclid Workshop
CANADIAN OPEN MATHEMATICS CONTEST (for top grade 11 and grade 12 students)