My 8-year old son also has ADHD. The best thing you can do to help him at school is to work closely with his teacher. I talk to or email my son's teacher everyday to see how he has been doing in class. Were there any behavior problems? Did he finish all of his assignments? Does he have any homework tonight? Are there any big projects due? If your son's teacher knows that you are concerned & he/she is willing to help out, they may make an extra effort to work more closely with your son.
On the other hand, if your son is taking medication and he is still distracted at school or while doing his homework in the evening, perhaps the dosage or what time of day he takes it needs to be changed?
If you can't talk to your son's teacher daily, perhaps you can make a behavior chart to give to your son's teacher to see if he is 1) finishing his assignments, 2) has missing assignments, 3) gets distracted in class, 4) follows the rules, 5) is cooperative, 6) works well with others, etc. You might have the teacher rate these things on a scale of 1-5. One might mean "needs a lot of improvement" and 5 might mean, "Doing well." The teacher can mark these things on either a daily or weekly basis. For example, if your son earns X amount of points in all of the categories in a day, then he can watch tv for an hour. If he earns XX points in a day, he can have a friend over. If he earns XXX points, he can earn $$ to spend. How you do the chart can be up to you & can be individualized for your son.
My son's psychologist recommended the book "Transforming the Difficult Child" by Howard Glasser & Jennifer Easley. It talks about how ADHD kids really need daily rewards, and daily praise. Instead of waiting until your child's report card comes out to praise him/her, praise your child daily for little things. Sometimes all kids hear is, "Stop daydreaming", "Stop talking", "You need to work harder", "Why didn't you turn in your assignment?" etc. Instead, tell your child, "Thank you for sharing your toys with your friend", "Thank you for helping me do the dishes", "I appreciate how you came right home and started your homework", "I'm proud of you for turning in all of your homework this week." Hopefully, the positive remarks will outweigh the negatives.
This book also talks about a point system. Your child can earn points for following the rules, using good behavior, doing chores, finishing his homework, etc. These points can be redeemed for "privileges", like watching tv, playing video games, going to the mall, having a friend over, going to an amusement park, seeing a professional sporting event, etc. If you child is "lazy" and doesn't want to do the work, then he won't have enough points to do anything fun. This plan works if you are consistent. Your child is ultimately responsible for working as little or as much as he wants. The more he works & accomplishes, the more rewards he will receive. It's a great book, is very easy to read & understand, it has lots of examples, and it works!
Lastly..if your son doesn't have an IEP or at least a 504 plan in the works, you should request an evaluation for these services at school. If he gets distracted in class, perhaps the teacher or resource specialist can make special accommodations where he can get tutoring or at least do his work in a room that is not distracting. You may be able to request taking extra copies of text books home in case your son forgets. Perhaps your son's teacher can give you a copy of his homework assignments for the week. Or, your son might even need written copies of your teacher's notes, so that your son can concentrate on the lecture & not concentrate on taking notes. The school may be able to help with study skills as well.
Sorry this is so long! Good luck!