Bismarck police are planning a "heavy" presence at Legacy High School after three bomb threats in two days.
Officers plan to conduct sweeps of the school building throughout the day. Student movement off campus also is being restricted.
The Bismarck Tribune reports belongings of guests in the building will be searched, and students' possessions might also be subject to search.
A threat mid-Monday prompted a shutdown of the school for the rest of the day. A threat early Tuesday afternoon prompted the evacuation of students, but they later returned to classes. A third threat was made late in the afternoon Tuesday.
The FBI is helping in the investigation.
The school is making counselors and crisis team members available to students.