Go here, and click on Education Store, and follow the links.
http://store.apple.com/ca/
It's not a flat discount, it varies from one item to the next, and varies from season to season; software that's just resold by Apple, for example, typically gets no discount.
Start of the school year is usually when the discounts are best. I got about $400 off my Macbook Pro (15% or so), plus a free iPod Touch. I know that the iPod Touch offer isn't on any more.
edit: I should mention that the prices on the educational store discount don't typically say how much is discounted. The best thing to do is open up two browser windows and browse both stores simultaneously, to see what's discounted. You'll see, for example, that most of the computers are discounted by 50 to 200 dollars, while ipods are not discounted.