Students and staff had to leave school in the afternoon when the second alarm came.
The school was searched, but the police did not find anything that could lead to injury.
The police are investigating illegal threats against groups.
Kungsbacka municipality announces that the school is open on Friday.
- We take this incident very seriously and will be on site during Friday so that everyone feels safe at school, says Hans-Jörgen Ostler, press spokesperson at Halland Police.