I agree with the previous posters, the 70-200mm f/2.8 IS and if money is no issue the 300 f/2.8 IS
I used a 70-200mm f/4L IS for graduation photos for my friends. I found the focal length to be adequate on a 1.6X Crop body for my purposes
The first day the hall had a lot of natural light coming in and I was able to get shutter speeds of around 1/160 at f/4 ISO 1600 which was enough to freeze action to a usable, but not perfect degree.
On the second day it rained heavily, and shutters dropped to 1/60 or so. I was in strife. It certainly showed in the results, I ended up underexposing and going to ISO 3200 to get anything meaningful. I thought that noise and exposure can be fixed in PP, but not motion blur.
Though I haven't used it, I recommend the 70-200mm f/2.8 IS, the extra stop is potentially very useful, and it's better to have it and not need it, rather than need it and not have it like in my situation. The sharpness advantage of a f/4L IS is useless if you are unable to freeze the action with it. Some might say "it's only 1 stop" but I found 1 stop often makes the difference between getting a shot and missing it.
A monopod might also help if the friends using your camera don't have very good telephoto technique, but I personally found subject motion was more of an issue. (Still do what you can to minimise overall blur)