No, probably not. I see drug deals happen at my school all the time, but I never do anything about it. I usually don't even give it a second thought since it's becoming so common. I would personally only give up my time to go to the police if someone were being seriously harmed. Anyone buying from someone else is doing so willingly and making their own choices.
Yes definitely, it's illegal and dangerous, if something bad happens due to the buyers being on the drugz yes its technically their own fault and they will buy them anyway elsewhere, but at least you know you did a good thing stopping it happening in school, younger kids might get involved that otherwise wouldn't have