A teenager was found carrying a knife after a brawl broke out between a group of students.
Police were called to Lightcliffe Academy in Hipperholme after staff broke up the fight and recovered the small weapon.
It happened last Friday but details have just been released.
Police and the school stress no-one was injured and that the incident was isolated.
It follows a spate of knife crimes in West Yorkshire schools in recent years.
A spokesperson for West Yorkshire police confirmed: “Police were called to Lightcliffe Academy around 11.30am on Friday, April 22, following reports of a fight between a group of pupils.
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“Teaching staff intervened and recovered a small knife from one of the pupils.”
Neighbourhood Policing Inspector John Simpson said: “Any incident where a blade is taken in to school is clearly concerning to students, parents, staff and the wider community.
“This appears to have been an isolated incident, and I must stress that occurrences such as this are rare in Calderdale.
“Officers will be speaking to those involved and continuing to work closely with the school, including delivering presentations on the dangers of carrying knives and weapons over the next few weeks.”
School principal Thomas Rothwell said the school take safeguarding issues very seriously and would not be commenting further due to ongoing police investigations.