Well,they do have many of the symptoms of ADHD. And the fact that this is happening at school as well as at home is an important part of the symptoms. You are a good mother to try and find out what is causing this behavior - and if they do have ADHD that would explain why your discipline methods have not worked.
The following is a good site with info on the symptoms (and lots of other things) on ADHD. Check it out - http://www.help4adhd.org/en/about/what/WWK1
If you think it might be ADHD or have further questions relating to ADHD, please post over here. http://www.medhelp.org/forums/ADD---ADHD/show/175.
; I am the CL over there as well and will be able to help.
Oh, its pretty easy to rule out thyoid issues (except for getting the blood part) and you might want to get that test just to make sure. Hope I will be able to help.