What is symbolic and ironic about Phoebe’s role in the school play?

While delivering a note for Phoebe to the principal’s office of her school, he sees that someone has written “Fuck you” on the wall by the stairs. This enrages him. Holden’s own language is often salty, and Phoebe asked him to stop cursing when he visited her in the apartment, but he finds this word especially abhorrent and does not use it around his sister. It upsets him that innocent children must see such a thing.