A Form One student was caught having sex with a teenager she met on Facebook a month ago, Berita Harian reported.
Her seven-year-old sister informed their uncle after seeing the boy in the girl's room in Terengganu at around 2pm last Friday.
Setiu OCPD Deputy Supt Ab Rahman Raja Mohamed said the 15-year-old student of a vocational school had gone to the girl's house about midnight.
"The young couple had sex twice before the incident was exposed," he said.
DSP Ab Rahman said the teenager tried to run away but was caught by the uncle, who sent him to the police station.