回复：烦恼妈，你儿子的信看得我直流眼泪。为你儿子心痛！

烦恼妈, I do think you need to show Principle your son's letter and make it clear it's the school's responsibility to educate students such behavior is not allowed. Especially, if there're incidence involved other kid who has to switch bus. The boy should be BANNED from any bus if such behaviour continues, and it's abosulutely the school's responsibility to condemn such comments.

You have your son's letter, the other kid's incidence, the bus driver's witness to make this a strong case.