Consumer Reports Health ( http://www.consumerreports.org/health/conditions-and-treatments/attention-deficit-hyperactivity-disorder/what-is-it.htm) did a report and said basically that while nothing has been proven it doesn't hurt to try fish oils (omega 3's). I have (in the 3 plus years on this site) never seen a post by a parent saying any additional diet change made a difference (if they had a normal diet). Having said that, omega 3's certainly wouldn't hurt to try!
I think that you would do a lot more good for her by learning more about things like ADD or ADHD. If she does have this problem, there is a lot that can be done for her at school and home. I suggest you buy the book, "The ADD/ ADHD Answer book," by Susan Ashley. It will give you a lot of good information. Best wishes!
Nordic Naturals is a good brand of Omega -3 Fatty acids. A lot sold in supermarkets, drugs stores, are rancid, actually doing more harm than good. They ahve a website, they also have research on their site as well.