Agree with lilred and kiwi. Is it just the weed or other stuff that has them concerned? I had a girlfriend who didn't smoke, she kinda minded and she wished I would quit but she didn't say too much to me about it cause my grades were a lot better than hers. (she wanted me to quit weed, I wanted her to stop falling asleep in the library lol) So if you wanna stop them from worrying just do better than them at school and then it'll contradict their whole smoking weed makes you dumb thing.

I like spite as a motivator. If someone tells me I'm dumb for smoking, I'll still smoke but work twice as hard as them and put it on them on every test just to prove a point.

Before you burn bridges, friends with the courage to talk to you like they do are very rare in life, friends who would be willing to make you hate them because they're looking out for your best interest are even more rare you should cherish them.