Talk to your teacher. The best way to get help after school is to ask your teacher to stay a few extra minutes so you can go over the topic. If she is too busy with other projects after school, ask to meet her either before school the next day or to make an appointment for when she is available next. Be willing to come in on the weekend, if that's a possibility for your teachers. Do this before you get too far lost in class.
Visit your school's learning center. Many schools offer learning centers with tutors who excelled in the classes you might be seeking help for. Sometimes teachers are on hand, but often the tutors are your peers, who know exactly what you're studying and what the teacher is looking for. If your school's learning center is on an appointment-only schedule, schedule as many appointments as you think you will need to get through the material that is giving you trouble.
Ask a friend for help. Barter with your friends for schoolwork. If your friend excels in calculus and you're practically fluent in German, do your homework together and quiz each other. Stay on task, though, otherwise your study time could easily turn into a gossip session.
Visit the library. If your school questions lie in literature or history, the library might be the best place to do your research. Journals, newspapers and encyclopedias are good reference tools for students.