Dude, I did really well in grade school despite all the issues caused by my shitty parents. I went out on my own and went to college and watched my grades go to shit. I quit smoking weed and saw my grades improve immensely. I finished college with mostly A's and B's.

Some people can go to class and smoke weed 24/7, but I'm willing to bet that most can't. I definitely can't, it makes me way too lazy. Think about your habits. As long as you read the material you're given you should be able to pass with a decent grade without trying to hard.