Lots of good information in the above replies but just wanted to add a couple of points:
1) http://wrightslaw.com/ Wrightslaw is a wonderful resource. As many others have said, *you* are your child's best advocate. ADHD/ADD is a protected disability under Federal law. Public school receiving federal funds are required to provide accomodations to a child diagnosed with ADHD in the same way that they are required to provide elevators for children in wheelchairs. However - some of these accomodations take time, extra work on the teacher's part and money from the school. While there are many wonderful teachers out there who actually want the best for the children in their class, others... welll... some of them, if you don't know, they won't tell you. So know what your child is entitled to, and ask for it.
2)Also, every state *should* have an organization out there to specifically to help you with this among other issues pertaining to kids with disabilities. In Arizona it's Raising Special Kids (http://www.raisingspecialkids.org/). They are free to parents, funded through state, federal and private grants. RSK does classes for parents on Section 504, IEPs, and how the process works. If you're not in AZ, there should be something similar in your state.
3)It's not going to be a viable option for everyone of course, but if you happen to be home anyway (i.e. maybe you're a SAHM or you work at home), it might be worth checking out a virtual school. We personally ended up going this route after struggling with the school system we were in for 6 yrs - I got laid off and given all the issues we were having, it made more sense for me to stay home than continue to fight with the school district over the 504 for my two ADHD kids. More info at K12.org. It's still a public school, you still *have* a 504, but many of the accomodations simply aren't needed anymore since you school at home on the computer - i.e., my kids didn't need an accomodation to allow them to move around or pace anymore, since at home they're not bothering anyone else and they can move all they want. We also didn't need the one for extended testing time, since all their tests are now untimed. Etc. etc.
Sorry about the length - hope some of this helps :)