What do you do when school contacts you?

My youngest is 4 and in UPK since September. Twice this school year the teacher has sent a message stating that he was unruly and not listening, throwing some stuff, but then eventually turned around. I don't really know how to reply? Each time I say I'm sorry and let me know if there's anything I can do and she says no need to apologize these things happen with 4 year olds.