A 19-year-old man was arrested by Hertfordshire police in Watford on Wednesday evening on suspicion of blackmail and making malicious communications.
More than 400 schools around the country were sent emails demanding officials cough up $5,000 or see their site blown up by an explosive device.
The schools are Abbots Hill in Hemel Hempstead, Valley School in Stevenage, The Sele School in Hertford, Duncombe School in Hertford, Stanborough School in Watford and St Albans High School For Girls.
A number of schools in other parts of the country have made similar reports to their local police forces.
Schools in London, Manchester and North Yorkshire were among those that received the email.
The Mirror reported that the email threat is written in both English and Arabic.
The National Crime Agency (NCA) said in a statement: "We understand parents' and teachers' concerns but stress there was no credible threat to the emails schools received.
"As this is a live investigation we are unable to comment further."
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