I've got good grades while using poppies everyday, tripping every week, and using the occasional benzo or other drug. Went to class on a medium dose of DOI once but it wasn't much fun. GPA for spring was 3.81, Summer I was 4.0. My cumulative GPA is not showing up, but if I worked it out right, it should be 3.86.
Most of the classes I took were easy, so that helped.
My tolerance to poppies has gone up a lot, if I don't get it down it may start to affect my grades. I plan on tapering down my use between the end of summer and start of fall classes. Maybe then I can stop using them every day. I'd use them before going to class, I think my grades would be worse without opiates.