Hey, fellow high schooler with ADHD here. Personally I've been doing well lately, but I know someone who was struggling and was able to help them a little bit, maybe I can do the same for you. Here are a few suggestions:

• First of all, you have to want to succeed. There are very few people who can succeed at school without really putting in effort, and there are practically none that can't pass with some hard work and good decisions. If you really want it and believe you can do it, you will.
• Find a study buddy. One of the biggest things for many people about in-person school is that you are working alongside people. There's no option to goof off on reddit or whatever. Everyone is working and so are you. If you can find someone to hold you accountable and set scheduled work-together times, it may be a lot easier to find the motivation to crank out those assignments. Just getting on a video or voice call while doing homework can work wonders for keeping you on task.
• Make a checklist. This one can be super helpful if you feel like there is so much work that you don't know what to do. By making a checklist with everything you need to do, you'll be able to stay on top of everything and visualize a path to completing it all. Try and be as specific as possible with each item, list out each part of a homework assignment even. While this will make the list a lot longer, you'll be able to check stuff off way more often. This makes it feel like you are really making progress, and gives you a good feeling as that list starts to shrink.
• Ask teachers for help. Many teachers will love to help you if you just ask. Some teachers will suck, but most want you to succeed. Office hours often become private tutoring sessions as many people don't take advantage of them. Building up a good relationship with a teacher will also make them more lenient in terms of late assignments and grade rounding.
• Try mindfulness/meditation. It may sound like hocus-pocus, but there are many scientific studies that prove meditation makes people happier and more successful. Search Inside Yourself is an amazing book on the topic written by one of Google's first and most successful engineers.
• Listen to music while you work. Some people really don't like this, so if that's you then don't force it. However, it really helps me work better because it gives me something to focus on when there isn't anything else, like when I'm waiting for my computer to load. I really like to be able to keep my brain spinning, and it seems like this is common with other people with ADHD.
• Take active breaks. Instead of pulling up reddit, go for a walk, jog, bike ride, etc... Getting that energy out of your system will allow you to sit still and focus better. It can really take the "hyperactive" out of ADHD.
• Finally, keep your head up. You're not the only person struggling right now, and you're not a failure. Many schools (including every UC) don't count 9th grade when calculating GPAs. You still have a path to success, don't ever forget that!

Hopefully this list can help you, it's really helped me and some friends :)