What I and almost every teacher I know usually say in this situation is, have you talked with your teachers? Many students don't and just wait for the worst to happen, or for the situation to get so bad the grade can't be rescued. Don't let that happen to you. Plus, teachers like it when students ask for help.
Now what you don't say is whether or not the semester is over. Have you taken the final? If not and you're just worried about it, again, talk to your teacher, or ask another student who is doing well in the class for help.
You can also:
1) Make a list of things you need to study for the final OR a list of missing assignments that they might still allow you to make up, and make yourself a study schedule to get them done, or to prepare for the test.
2) Find out WHY you are failing. Is it lack of effort? Not understanding the material? No time during the week to study? Once you figure out why this is happening (and in more than one class?) then you can figure out how to help yourself.
The most important thing is DON'T WAIT! Get help tomorrow, or talk to a guidance counselor who can set up some conferences. You can't get out of this hold until you stop digging and start climbing. Hang in there!
See other tips from Seven Habits for Effective Students at the link below.
Follow these rules:
1- Study as hard as you can.
2- Don't listen to anyone who tries to discourage you.
3- If you get bored, try to do something else but don't delay your study for a long time.
4- If there is a problem with a certain point, don't be angry. Focus on the problem and take your time.
5- Challenge can be good and bad at the same time. For example, if you plan to challenge someone just to get more marks than s(he), don't get upset if you did not. If you did, however, don't talk about it and think of it as a way to make you succeed.
If you follow these rules, you'll be able to pass absolutely everything and with time, you'll see that study has never been easier.