“Chartwell Manor,” the veteran cartoonist’s harrowing new graphic memoir, recounts his two years at the Mendham, NJ boarding school run by headmaster “Sir” Terence Michael Lynch — a serial sexual abuser who manipulated young boys into “cuddling sessions” after fondling and beating their nude bodies. Head’s “bland, suburban” existence was shattered at 13 when he scored subpar marks in the 7th grade.