For six months, Adriana Lim Escano’s teenage son went to school and said nothing. He had tried to do the right thing – stepping in when a group of boys bullied a classmate – and paid for it with half a year of misery, name-calling and social isolation.

His mother only found out when another parent called to say her son had voiced suicidal thoughts to a friend.

The school’s response, when it finally came, was a talking-to from the discipline committee. No suspensions. No meaningful...