Read everything assigned. Teachers know when you haven’t.
Do the homework, but don’t share the answers. Sharing answers outside of class is just as bad an idea as sharing them in the classroom, and doing so is not undetectable.
Learn the vocabulary as fast as you can. At the high school level, you’ve got to learn between 7-8 new words per day. That’s roughly 35-40 new words per week. And they’ve got to become part of you. Not just words you can access and remember. Words you use. So you have to use them. Try writing them into sentences with words that you already know, even if every other word in the sentence is in English. Try drawing pictures for objects and for things that you do. Every time you brush your teeth, spend 30 seconds going over old words and a full minute with the new ones. Change your passwords frequently with Spanish vocabulary words. Close your eyes and picture the words in your head. Above everything, say the words!
Do what you can to learn the accent rules. A 4th year student that won’t bother to learn a few accent rules won’t see an A.
Listen to Spanish pop music. It’s the best way to pick up some short, expressive sayings. It’ll keep you current and it’s fun.
You need more than an hour of background input. Play your favorite movie with the Spanish audio track on, over and over again in the background. Do this at least once a week. After ten times, switch to another movie that you already know. Revisit your favorite movie after two months. This will sharpen your ear and increase your pronunciation ability.
Don’t give up. Don’t allow negative or disparaging talk or thoughts. Even at home. You can do it.