The following links will connect you to sites that have copies of old contests with complete solutions. There is also information on registration and results.
Canadian Math Contests: This is a link to the Gauss, Pascal, Cayley, Fermat, Euclid, and Canadian Senior and Intermediate Mathematics Contests. You will find old contests with worked out solutions.
American Mathematics Contests: This will link to the Grade 10 and the Grade 12 American Mathematics Contests. You will find the answers to old contests as well as practice questions.
Many students receive recognition for their excellent results on the Mathematics Contests. These come as Certificates of Distinction and as awards for the top placing student in the school. The Certificates are presented in class and the awards are presented at the Awards Ceremonies in late June.