A telltale hickey has made things sticky for the coach of a girls volleyball team at Poly Prep Country Day School, a fancy private school in Dyker Heights. Coach Lisa Guttilla, 37, has been charged with misdemeanor sexual abuse and endangering the welfare of child, an unidentified 14-year-old girl on her team. Their affair was discovered after the girl's mother demanded to know the origins of a hickey on her neck, and the girl confessed. Guttilla, a part time employee at Poly Prep whose photo we're still trying to find, "engaged in sexual conduct" with the girl on at least three occasions, police sources tell the Post.