Say NO to friends who try to get you to do something else. If they are your friends, they will respect you for your diligence. Don't worry about how much time you have, just get it done. For instance, if you have an hour between classes, that might be a good time to go to the library and do some reading. Determine the time of day that is best for you to study.
Some people are morning people, others do their best thinking in the evenings.
Study when your mind is the most sharp. Review the previous day's notes before going to class. You will be better prepared to participate in class discussions. Review your notes on the evening or afternoon after your class. This will move the material from short term memory to long term memory.