Ohh girl! I guessed your age just by reading this question(below 18)! Its good to smile. Guys love the girl who smile, laugh and are very fun to talk to rather than talking to a girl who is morose and sad all the time! Keep smiling!
Dont worry about it. You do need to control it sometimes like focus on the lectures so you are serious and look like you are paying attention when someone is talking to you. I am know as the shy quiet girl but I smile all the time. But there are only a few people like my boyfriend that can tell if my smile is actually happy or masking pain. I would not stop just control it when you nee to me serious.