why does valentine think the I.F ia at her school. what does graff really want

When Valentine sees the I.F. Marines outside of the school, her first thought is Ender. Her second thought, however, is that they're there for Desmothenes. She knows the I.F. has the capability to track her down and is afraid of being exposed. Luckily, Graff wasn't there to take her alter-ego, he was there because he needed her help with Ender.