Ways to pass classes

Priority.
Make school a priority. If you are only studying occasionally, chances are you will fall behind in your class. Some subjects require you to know certain things to continue to comprehend what the teacher is teaching. And if you haven't been learning that, you will fall behind.

Join a study group.
If you don't understand something that is going on in the class you will probably find someone who does. That way they can help you with your trouble spots and you can help them. It's a win-win situation.

Set aside study time.
Don't only study if you have time, you need to make time for it. Whether that means missing a movie with friends or watching television. In the end you will be happier because if you fail a class you are just going to have to take it again.

Talk to the teacher.
If you are truly struggling with a subject and are studying and still not getting it, talk to the teacher. It will show that you really are trying to make an effort. The teacher can maybe point you in the direction of other helpful ways to study.