In the classroom, you should be ready to learn and not view it as a time to laugh at jokes. Sit in the front row, if you don't have assigned seating. Avoid classmates who like to joke around in class. This attitude will sink into your study time. Concentrating on one subject for an entire study session can be counter-productive and vary on what subject you are studying. This will freshen your concentration by fixing your eyes onto a new topic. It can be the hardest thing to do in today's world of distractions. If you begin to feel like you're not being as productive, say to yourself, "be here now." Then slowly focus back to what you were studying.  It might sound gimmicky, but reminding yourself in this way could help. It doesn't work for everyone.  Say this with a deep breathe while your eyes are closed for a deeper calming effect.

TL;DR in Vietnamese:
Hoàn toàn tập trung. Thay đổi đề tài. Tập trung vào hiện tại.