It happened at St Aidan’s High School and involved a male wearing a grey cap with a red logo, a black jacket and blue jeans.
He is believed to have walked into the bike shed at St Aidan’s before taking a Carrera Vengeance mountain bike by snapping the bike lock.
Police are requesting the public’s assistance to help establish the full circumstances surrounding the incident.
In particular, they are appealing for information about the male involved.
It happened between 4pm and 6pm on Tuesday 17 April 2018.
Anyone with information that could assist the investigation should contact North Yorkshire Police on 101, select option 2, and ask for Ben Widdowson. You can also email firstname.lastname@example.org
If you wish to remain anonymous, you can pass information to Crimestoppers on 0800 555 111.
Please quote the North Yorkshire Police reference number 12180065539.