Hi, I am also the CL on the adhd site here - this is really pretty common with adhd kids. Part of the reason may be because her meds are not lasting long enough. The rest of the reason is because this is how adhd kids work. I have several really good links that will help you both at school and at home. I'll dig those up and get back to you. Is she on Ritalin or another med?
I am not sure that you can teach a 5 year old to "not pay attention". However, 5 year old boys are not great at paying attention anyway. And, ADHD is genetic so you might want to keep an eye on him if his teachers start complaining.
You said she, "has been doing this for the last couple of yrs." If she has been on meds the whole time, thats a pretty good sign that she is not getting enough meds. Also keep in mind that each med has a specific duration. Hence the reason I asked what she is on. Ritalin for example lasts about 4 hours tops. Once the duration is done, the child reverts back. If she is at home, and the med is over, then you must expect a typical ADHD behavior. Which she is definitely showing.
The following link is on paying attention. The site it is from has many, many good ideas that will help you. Just go to the Parenting ADHD Children on the top of the page and look at the topics. The link is ...
Hope this helps. Please post if you have more questions or need more info.