This KBCN has been withdrawn and is no longer valid.
This is because Building Bulletin 101 was updated in August 2018.
KBCN withdrawn on 04/02/2019: The web page that hosts the currently active version of Building Bulletin 101: ventilation for school buildings, available here, states that the document was published on 11th March 2014. However, when opening the document itself, the publishing date refers to '5th July 2006'. The current version of the standard which should be used is Version 1.4 dated 5th July 2006 within the document.