Don't Tell, Don't Tell, Don't Tell — Liane Shaw

Sixteen-year-old Frederick has a lot of rules for himself. Like if someone calls him Freddy he doesn't have to respond; he only wears shirts with buttons and he hates getting dirty. His odd behavior makes him an easy target for the “Despisers” at school, but he's gotten used to eating lunch alone in the Reject Room.
Angel, in tenth grade but already at her sixth school, has always had a hard time making friends because her family moves around so much. Frederick is different from the other kids she's met - he's annoyingly smart, but refreshingly honest - and since he's never had a real friend before, she decides to teach him all her rules of friendship.
But after Angel makes a rash decision and disappears, Frederick is called in for questioning by the police and is torn between telling the truth and keeping his friend's secret. Her warning to him - don't tell, don't tell, don't tell - might have done more harm than good.