Just flat out ask somone from your schools smoker group "Hey who here will sell me weed" That would probably work fairly well. Although the school I went to was described as "The Greenhouse". But yeah other than that Just slip the guys you get stoned with a 50 note and if thats not all cool they're either wankers. Or might be some initiation/test.
Follow your nose... I guess I'm fortunate to live in California where I just went to MediMar and got my card to get medical pot. I got several outside sources too..even got some free chronic plants once from one. It's the most popular recreational drug after alcohol in this country, so it shouldn't be hard to find at all... It sounds like you just need to go out to make some connections and ask around. If anything, find out where the pot or other drugs are in your area and go from there..but I wouldn't go online. You'll probably end up getting low-grade stuff and/or ripped off. Learn all you can about it, including strains, quality, prices, and amounts...there's a lot of kick-ass pot out there nowadays, but unfortunately also a lot of crap, and it's easy to get ripped off if you don't know what you're looking for. Look for trichomes - those white crystals of THC on the buds..the more you see, the more potent it is. Also, the smell is important..if it smells like dirt or manure then it's low potency. If it has that strong fresh plant smell (like medical-grade), then you know it's potent. Good luck!