It helps if these study groups serve to review material already learned or to clarify problem areas. Make sure everyone is motivated. After the test, reward yourselves with pizza and conversation. Start studying from the first day. Be ahead in the reading material.
Some classes require more work or effort than others.
For example, math and foreign language are subjects which require daily work. Because these classes require knowledge of basic material which then builds, if you don't keep up you'll be lost. It does not mean passively reading some material. It does not mean endless hours spent in a frustrated search for understanding. Studying involves steady progress.