Abbot’s Hill School received a bomb threat on Monday morning.
The girls’ school in Bunkers Lane was one of six schools targeted across Hertfordshire.
Police say that there is “no evidence to suggest these threats are credible”.
The other affected schools are Valley School in Stevenage, The Sele School in Hertford, Duncombe School in Hertford, Stanborough School in Watford and St Albans High School.
A Herts Police spokesman said: “A number of schools in other parts of the country have made similar reports to their local police forces.
“We take incidents of this nature extremely seriously and an investigation is underway.
“Anyone with information that may assist the investigation is asked to contact Hertfordshire Constabulary via the non-emergency number 101, quoting ISR 172 of March 19 or report information online at www.herts.police.uk/report
“Alternatively, you can contact the independent crimefighting charity Crimestoppers, anonymously on 0800 555 111, or through their anonymous online form at www.crimestoppers-uk.org
“No personal details are taken, information cannot be traced or recorded and you will never need to go to court.”
The school was unavailable for comment.