Kendall offered some excellent strategies. One more is to see if you can stand up or move around in the back of the classroom during lectures. Getting stimulated by moving will help you pay more attention.
The real issue is that, if you do have ADHD, the ADHD brain is motivated and focused when there’s interest and urgency, NOT when something is simply important. That’s why your attention fades quickly when you’re bored.
My son is in 10th grade and the same thing happens. He hates school because he says it’s boring. His best grade right now is in psychology class, because he finds it interesting (and because it’s an online class where he can do the work at his pace and move on, not sit through lectures or spend extra time on things he already knows).
You might try a wearable like a RE-Vibe. It vibrates on particular intervals to remind you to self-check if you’re on task.
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism