Anti-vaxxers threatened to launch a protest outside a school after wrongly believing staff would "lock in children and vaccinate them".
The school was sent "unacceptable" messages over social media, which included personal threats to the principal.
Staff have since hit back after their "emergency planning operation" was misinterpreted, Nottinghamshire Live reports.
Ellis Guilford School, in Old Basford, Nottingham, had planned a "critical incident procedure" on Wednesday, May 12, where it would prepare for a serious incident.
The school's principal, Chris Keen, sent out a letter to parents letting them know that the site would be locked down during the event, in what the school described as "similar to a fire practice".
But when using the term 'lockdown' in a letter, some in the community believed the school was planning a Covid lockdown procedure, sparking a furore.
Mr Keen said one anti-vaccination protester had made a YouTube video making false claims about the letter.
"Unfortunately the letter I sent to parents about this practice found itself in the hands of some anti-vaccination protesters, and there are a number of them around," he said.
"There was some correspondence on Facebook that was unacceptable, there were some threats including a personal threats to myself, and threats of a protest taking place outside the school.
"There was even an individual of one of these sites who created a YouTube video about the letter itself.
"I sent a further letter believing that people were misinterpreting the word 'lockdown'.
"Obviously the word 'lockdown' now takes on a different connotation for all of us, so I tried to explain in different language what it was and the importance of it.
"As a result of that, it didn't stop any of these challenging Facebook messages."
Mr Keen and leaders in the Creative Education Trust took the decision to postpone the event until a future date, fearing a potential protest outside its premises.
The police were also initially involved when the protest was planned but it has since been confirmed by the force that no protest will take place.
Speaking on cancelling the procedure, Mr Keen added: "To be quite frank I had to make a judgement call.
"They knew it was going to take place tomorrow, Wednesday May 12, and the safety and wellbeing of all pupils in school is of paramount importance.
"We don't want disturbances, we don't want interruptions and we don't want individuals coming to the school who are not welcome.
"We will carry out this practice in an unannounced way, just as we do a fire evacuation.
"There are no implications of not doing it now, we think it's important as do many schools throughout Nottingham.
"It has just drawn a lot of attention from certain individuals who are very against this, and think somehow we are going lock the children in the school and then vaccinate them."