Schools in Peterborough and Cambridgeshire have been put on alert after staff reported that they have been sent "threatening" emails.
Police have confirmed that forces across the country have reported similar incidents.
The emails are believed to have informed schools that they will be targeted in an attack later today.
A Cambridgeshire Police spokesman said: "We are aware of a series of malicious communications to schools across the country this morning (Wednesday 28 March).
"Enquiries are being carried out to establish the facts and forces are working together, along with the National Crime Agency, to investigate who is responsible."
The emails say that at 3.15pm a car will drive into as many students as possible as they try to leave.
The emails read: “If you try and evacuate them the driver will get out and shoot any student leaving.
“You have a choice here, you can ignore this email and risk the lives of the students you say you care for, or you can listen to what we are telling you.
“The only way out is to go out with a BANG.”
The email is signed off: “From your team PI3x13t.”
Some schools are allowing parents to collect children early as a result and are contacting parents and carers.
The Peterborough Telegraph understands some schools allowed children to be collected at 2pm and parents have reported on social media schools including, Yaxley Primary, Guyhirn Primary, Fourfields Primary in Yaxley, William de Yaxley, Discovery, Eastfield Primary, St Ives, as well as Priory Infants and Longsands Academy are among those reported to have had the email.
Werrington Primary, Sawtry Infant School and Sawtry Junior Academy did not the email we can confirm despite earlier speculation.