There is seperate legislation in place for schools. No knives allowed, except for in canteens or lessons that require it such as art,dt,cooking etc. The caretakers house is also exempt.
Scout groups and the like using school premises are exempt too for pioneering etc.
That's from memory, Id need to get out blackstones for the actual wording.
I think general common sense would apply.
Criminal Justice Act 1988 section 139A
139A Offence of having article with blade or point (or offensive weapon) on school premises.
(1)Any person who has an article to which section 139 of this Act applies with him on school premises shall be guilty of an offence. .
(2)Any person who has an offensive weapon within the meaning of section 1 of the M1Prevention of Crime Act 1953 with him on school premises shall be guilty of an offence. .
(3)It shall be a defence for a person charged with an offence under subsection (1) or (2) above to prove that he had good reason or lawful authority for having the article or weapon with him on the premises in question. .
(4)Without prejudice to the generality of subsection (3) above, it shall be a defence for a person charged with an offence under subsection (1) or (2) above to prove that he had the article or weapon in question with him— .
(a)for use at work, .
(b)for educational purposes, .
(c)for religious reasons, or .
(d)as part of any national costume. .
(5)A person guilty of an offence— .
(a)under subsection (1) above shall be liable— .
(i)on summary conviction to imprisonment for a term not exceeding six months, or a fine not exceeding the statutory maximum, or both; .
(ii)on conviction on indictment, to imprisonment for a term not exceeding [F4four] years, or a fine, or both; .
(b)under subsection (2) above shall be liable— .
(i)on summary conviction, to imprisonment for a term not exceeding six months, or a fine not exceeding the statutory maximum, or both; .
(ii)on conviction on indictment, to imprisonment for a term not exceeding four years, or a fine, or both. .
(6)In this section and section 139B, “school premises” means land used for the purposes of a school excluding any land occupied solely as a dwelling by a person employed at the school; and “school” has the meaning given by [F5section 4 of the Education Act 1996]. .
(7)In the application of this section to Northern Ireland— .
(a)the reference in subsection (2) above to section 1 of the M2Prevention of Crime Act 1953 is to be construed as a reference to Article 22 of the M3Public Order (Northern Ireland) Order 1987; and .
(b)the reference in subsection (6) above to [F5section 4 of the Education Act 1996] is to be construed as a reference to Article 2(2) of the M4Education and Libraries (Northern Ireland) Order 1986.]
S. 139 defines these articles as
Subject to subsection (3) below, this section applies to any article which has a blade or is sharply pointed except a folding pocketknife. .
(3)This section applies to a folding pocketknife if the cutting edge of its blade exceeds 3 inches
Thats the law on knives in schools