Pot causes short term memory loss only during the high. Regardless of what you're often told, the studies only show short-term memory (but not long-term memory) is impaired as long as the user is high and no longer. So even if you smoked up right before the big test it probably wouldn't affect you much since the long-term memory is mostly what's important then. But try not to go to class stoned if you're afraid you can't do it without forgetting stuff (me, I go stoned anyways, and bring a notebook ) and don't smoke when you should be studying instead.