Study every day--I'm not saying you have to spend hours every night studying, just that you have to study some. I would recommend keeping your vocab flashcards in your backpack and running through them once per day. It may seem like a lot, but you can probably finish them all while waiting on the bus or some other downtime you have during the day, and if you look over the words daily, the time you spend studying will actually decrease. It takes a lot more work to cram than to just spend a few minutes per night and truly know the material.
Don't retire what you've learned--One of the biggest mistakes kids make is thinking last chapter is irrelevant and its material can be forgotten after the test. This isn't true in any class, but least of all in a foreign language. If you don't believe me, go back and reread this paragraph, ignoring any word you learned more than 5 years ago. Yet, many students think words they learned 5 days ago are no longer needed. In reality, once you learn a word in class, you need to keep reviewing that word until you know it so well you can't forget it.
Do your homework--This one speaks for itself, but every year I'm shocked at how many points are lost because of zeros on homework. I’ve even had kids fail my class, simply because they lost too many points from not doing homework and found themselves on the wrong side of the 70. Additionally, many teachers get the homework from the same company that makes the test banks they use. Don't be shocked if you see similar questions on homework and tests.
Practice with a friend--Language is social. The only reason it exists is so people can communicate with each other. You learn best when you practice with other people. It could be friends in class, native speakers you know, or following native speakers on social media--but find someone to chat with and use the language for things you really care about. The topic isn't as important as the practice. Just talk.