If you look up disorders or read about disorders you will understand the situations that are happing its also importain that the teachers know what his disorder is
The sad thing is that at his age there is not a lot that you can do at home to affect his behavior at school. Yes, there are things (behavior therapy) that can be done at home, but since at this age immediate consequences work best - a lot is on the school.
Hopefully, your doctor gave you techniques that work with adhd kids. Ya, medication is one thing, but there is a ton of stuff that can also be done over the summer to make both of your lives easier. If not I suggest that you immediately order "The ADD/ ADHD Answer book," by Susan Ashley. I guess the good news is that now hopefully you understand why your son has not been behaving.
I am curious, has he been having troubles all year - or only at the end of the year?
Also (probably not time this year), but early next year you want to request a school guidance team and get a 504 or an IEP for your son. It will help him and the teachers. If you are not sure what those are - they are explained in the book I recommended.
While you are waiting for the book, it wouldn't hurt to go over to the ADHD forum and read some of those posts. See you there!