It's really not that difficult to imagine that someone with ADHD or ADD might have trouble with school. Between having to focus for a long time to having to sit still for what feels like even longer, ADHD and school just don't seem to mix.
However, there are quite a few things ADHD students can do to improve their school results. One of the things is to change the way they study. Most ADHD students do better while studying in a quiet place, free from distractions. If they study at the same time each day, they are more likely to be able to focus during that time, as they will have trained their brain that this time is time to focus.
Another study tip for students with ADHD is to find a specific study technique or system and follow it religiously. One of the best systems for this is the Pomodoro Technique, which involves using a timer set for 25 minutes to regulate study time, followed by a brief 5 minute break to refresh.