Being at school doesn't have to keep you from expressing--in a school-appropriate way--your love and affection for your boyfriend. Let him know subtly, in little ways, that you're thinking of him and happy to call yourself his girlfriend. Just remember to follow all school rules when communicating your devotion.
In His Locker
Many guys may not admit it, but they'd love to find a sweet surprise in their locker from their girlfriend between classes. All at once, you'll brighten his day, remind him of your love and bring a smile to his face. Depending on what your guy likes, slip a gift inside his locker when he's away. (Ask for his key or combination beforehand and tell him you have a surprise for him.) It could be a short love note, a candy bar with a ribbon tied around it, his favorite soft drink, or--if he gets to leave campus for lunch--a small gift card for a nearby fast-food restaurant.
In Class
It can be a little challenging to be affectionate during class, but if you pay attention and get creative, it's possible. If you're working in groups, catch his eye now and again and give him a sweet, genuine smile or wink. If the teacher calls on him and he seems a little stuck on the answer, help him out: raise your hand and throw in your two cents to "save" him. If you're in a group situation together, be supportive of his ideas. Passing notes can be risky, so stick to things that will build him up without breaking the rules.
At Lunch
Lunch is the time that most girlfriends want to monopolize their boyfriend's time, but you can score points by resisting this temptation. Instead, chat with him in the lunch line and give him a compliment about his clothing or something he did in class. If he's had a rough day so far, pay for his lunch. Then, depending on your school's rules regarding male-female contact, give him a quick peck on the cheek or a pat on the arm and tell him to go chill with his buddies. Low-maintenance and sweet are two qualities that many guys believe make a great girlfriend. When the bell rings, walk him to class (but don't be late to your own) and slip a short, encouraging note in his pocket.