Listed here are some of the activities in which MacLachlan students vigorously and successfully compete, several in which they have earned top honours:
- The Fulford Debating League
- The Nora MacRae Public Speaking Tournament
- The Independent School International Public Speaking Competition
- The Ontario Student Debating Union
- The Kiwanis Festival Choir Competition
- The Battle of the Books
- The Festival of Oral Poetry
- The McMaster University Science and Engineering Fair
- University of Waterloo Mathematics Competitions: Gauss, Pascal, Cayley, Fermat, Descartes, Euclid
- The Great Canadian Geography challenge
- The Commonwealth Overseas International Essay Competition
- The Cambridge Calibrated Mathematics Test
- The Canadian Mathematics Competition