I think, for story's sake, I'm gonna say that Tess was on her way to the school when she was hijacked...so there's a tie-in for her to keep going there regardless of the outcome of all this.
And I think it works with her backstory too.
So she'll ferry whoever's going over there, get an update on current events, then pop back to tell Magneto what's going on. Then come back to help out.
Her story will be pretty close to what actually happened to her, except she'll just say the bus broke down or had an accident, rather than tell them Magneto stopped the bus because he A) needed some help, and B) was aware the school might be compromised right now, and didn't want a promising young mutant to wander into the tiger's mouth.