Unfortunately, the 100-400L IS is going to be too slow for night
football. If that was the main purpose for buying that lens, you
might want to return it and get something faster. F/2.8 is an
absolute minimum for shooting under the lights. I use a 70-200
F/2.8 with some success. I also use a 100-400 for daytime games.
I would love to try a Canon 200mm F/1.8 lens for nightime football,
but at $3500-$4000, I'll shoot at F/2.8.
Another option is flash photography. First, check to see if flash
is allowed, them look into a "Project-A-Flash".
http://www.leppphoto.com/Project-A-Flash/index.html
Good luck.
JimD