The all clear has been given at Palm Beach Currumbin State High School, which was sent into lockdown twice on Wednesday morning.
It follows a cleaner's discovery of of white powder in an envelope, with a threatening letter around 5.30am.
That powder examined by QFES experts who declared to be harmless about 9.30am.
Students were evacuated from their classrooms for a second time shortly afterwards as police searched the campus for explosives.
The scene was declared safe shortly after 11am, and most students have now gone home.