Prepare for class ahead of time, reviewing notes from the previous lecture. Read textbook material to anticipate the upcoming lecture.
Resist and eliminate common distractions. Sit closer to the front of the room if possible. Move away from students who like to chat during class. Learn the spider technique for removing distractions. A spider focuses on catching a bug in its net. A vibrating tuning fork distracts them, but the spider quickly ignores the vibration. Practice blocking out coughs, hiccups and other physical movements and focus on the instructor.
Shift posture in your seat. This keeps the blood circulating. If your class has a break, walk out of the classroom. Stretch your legs and arms.
Say the words "Be here now" when your mind starts to wander. Commit to saying these same words consistently for an entire class. Our mind often wanders due to worries or thoughts we want to focus on. In addition to saying "Be here now," begin setting aside a specific time each day outside of class time, maybe 30 minutes, for worry and think time.
Research other ideas and exercises for improving your concentration. SuccessConsciousness.com gives many tips on improving your concentration.