This is good advice Takahiro. There is another option for keeping up with the kids other than a fast focus and fast glass. Remember the old photojournalist saying: "f8 and be there". This style may yield more keepers during those fast paced moments. I understand that the 7D has very good noise control so f3.5-f8 and high ISO might be an option. Also many of these shots will probably be printed small so cropping and a little bit of noise shouldn't cause too many issues. The 35mm sigma would be good on something like the 7D because once again cropping won't be a big deal for school magazines etc.