Nah, much more luck to be found on premed101.com , the Canadian forum here is especially dead.
As for the OP's question, I wouldn't if you can help it. As a general rule for your 3 LORs you want one academic (prof, PI), one from a volunteering-type activity that can speak to your interpersonal skills and compassion etc. etc., and one wild card.
Even if this is your 3rd letter, what can your high school teacher say about you? Anything in the academic sphere wouldn't carry much weight because it's from when you were 16-17 years old and they can hear about your capabilities as a student from someone who taught you in university which is much better. If it's not academic in nature, then you're better off with another letter from someone who has interacted with you during a volunteer or EC activity in undergrad. If your 3rd letter would otherwise be extremely generic (I'm Professor X and Jeff took my Physiology class and got an A+ he seems smart) then I'd go with the high school teacher, but if there's anyone else that could write a flattering letter I wouldn't risk it.