From your first day at Western, you'll notice that it is a party school. People drink their faces off during Orientation Week (both frosh and upper-year students), and student life revolves around the weekend. There isn't pressure to drink, but be warned: nearly everyone does. Playing beer pong, going to a kegger, or even drinking at a bar is a staple of the University experience, and these actions definitely do not fall short at Western. Underage drinking isn't taken very seriously. Minors caught using a fake ID at a bar might be thrown into a drunk tank, but that's about as stringent as it gets. In residences, you may get warnings; however, it's rare for students to be kicked out of residence for drinking unless they were destructive.
Marijuana is prominent, but there isn't pressure to partake. Western's inclusive community allows "to each his own," so if weed isn't your thing, don't bother. If caught blazing in a residence, the consequences would most likely be far more severe than underage drinking.