Officials say the police presence is to reassure students and parents that Bridgewater-Raritan High School is safe.
Principal James Riccobono addressed the 2,900 students before classes began Monday. The principal told them that acts of violence can happen anywhere and he praised a student for alerting authorities.
Authorities say a 16-year-old junior was arrested Thursday after a police resource officer at the school was told the teen was planning an attack.
Authorities say they found shotgun shells, bullets, black powder and explosive devices in the teen's home.
Officials also arrested a 17-year-old Immaculata High School senior, who is a competitive target shooter.