A lot of it comes from a teacher's willingness to accomodate a student who has been diagnosed. I am not saying that if a student who has ADD/ADHD is making bad grades it is all the teachers part, but the teaching style can make a big different. Almost every child I have come across with either one has been very kinesthetic and needed something to do with their hands to stay focused. Sometimes even being able to stand up every once in awhile helps because they get to move.
One thing I have seen done is giving a child something as simple as silly puddy. Most of the time, they can sit there and fidget with it, which helps get some of that antsi-ness out, but it's not distracting enough to take away from what they need to be learning. Sometimes just having some way to physically move will help them focus better.
If they have bad grades, it's not because it is impossible for them to obtain higher grades, it's a matter of how they are placed in the education system. A teacher cannot expect an ADD/ADHD child to sit in a rigid environment without moving a muscle and actually do well and stay attentive.
One thing I do at work with some of my girls who I know have ADD is that I involve them in the teaching aspect. I make them my "special helper" and have them do things like pass out papers and supplies or help somebody who may need it. Putting them in a small leadership role also helps them to focus because more is being expected of them, but in a different sort of way.
That's what I have found to work for me.