I looked yesterday and the trac-phones or go-phones and they aren't that much cheaper than adding our daughter to our plan. But, it has no contract so if it's abused we just drop it and it will allow us to make it an "emergency" only line, and there is no chance of all our minutes getting used. We did this with our son when he was an early teen. He mowed lawns in the summer so I wanted him to have it for safety and to call me for a ride but no other use. He was very good with his, he only used it like we asked. I really expected it to be an issue but I think he knew we were serious so he actually obeyed that rule. Nearly every kid in my daughters 5th grade class has a phone...makes me ill sometimes.
We are still tossing around this whole go-phone thing but in your case it would be a good idea for her to have.