A primary school class in the Harrogate District was evacuated after an eight-year-old girl pulled out a knife last week and "made threats" to staff.
Details of the incident emerged in a tweet by a Police Community Support Officer, who said when he arrived he was then threatened.
"The child told me they would stab me in the heart," tweeted Matt Murphy. "So this week we are educating people about the dangers of knives."
North Yorkshire Police said the girl is "now receiving appropriate support".