Six Leeds high schools received telephone calls making apparent bomb threats on Monday morning.
Police attended all six schools and were satisfied the threats were hoaxes. No suspicious packages were found, and none of the schools were evacuated.
Police said schools in other areas of the UK had also received threats.
Chief Superintendent Paul Money, Leeds District Commander, said: ““We will continue to offer all necessary support and reassurance to the schools and the wider community.
“These malicious calls have the potential to cause significant disruption to our schools and unnecessary alarm to staff, pupils and parents alike.
“A full investigation is underway into the source of the calls and we will be liaising with colleagues in other parts of the country that have experienced similar incidents today and previously.”
A spokesman for Leeds City Council said they were aware of the threats and were liasing with police officers to support the school.