You can prepare for school, even if you have only a few minutes to spare.
First, start to get up as early as you need to go to school. If your bus arrives at 8:30, you should give 60 - 75 minutes to get ready and eat a good meal.
If you're waiting for a school bus or someone to take you to school, crack open the books that you're going to read from. Preview the material and try to learn the vocabulary of your courses. If some concept looks intimidating, keep a note about it and ask a teacher, parent, or someone you trust to explain it. Or google it! When you have homework, make it a priority to get it done. If it is hard to score high on your homework, don't worry about it. Just do your best. But do keep a record of your mistakes and errors. The earlier you're able to identify your mistakes and errors, the earlier you can take action to correct them. And this can help your confidence.
Confidence is good. But remember that it's not likely that you'll score high in every subject. It's typical to have a mix of strengths and weaknesses. And consistently taking time and effort to study and learn - even if you don't meet your ideal goals - builds character, which is key to success!