how does the LA justify giving a place to a male when under 18s cant have a GRC?
If this is to do with provisions under the Equality Act 2010 (I don't know if that is the case, just assuming) then acquisition of a GRC is not relevant because the EA2010 Protected Characteristic of Gender Reassignment includes school children.
EA2010 does not specify a minimum age limit for the PC of GR:
7 Gender reassignment
(1)A person has the protected characteristic of gender reassignment if the person is proposing to undergo, is undergoing or has undergone a process (or part of a process) for the purpose of reassigning the person's sex by changing physiological or other attributes of sex.
(2)A reference to a transsexual person is a reference to a person who has the protected characteristic of gender reassignment.
(3)In relation to the protected characteristic of gender reassignment—
(a)a reference to a person who has a particular protected characteristic is a reference to a transsexual person;
(b)a reference to persons who share a protected characteristic is a reference to transsexual persons.
https://www.legislation.gov.uk/ukpga/2010/15/section/7
Section 85 mentions pupils with the PC of GR:
85 Pupils: admission and treatment, etc.
- In the application of section 26 for the purposes of subsection (3), none of the following is a relevant protected characteristic—
(a) gender reassignment;
(b) religion or belief;
(c) sexual orientation.
https://www.legislation.gov.uk/ukpga/2010/15/section/85
EA2010 Explanatory Notes mention that transsexual pupils are protected from discrimination:
EA2010 Explanatory Notes
Section 85: Pupils: admission and treatment, etc.
- This section is primarily designed to replicate the effect of provisions in current legislation applying to schools. In addition, it extends protection from discrimination to transsexual pupils and pupils who become pregnant.
https://www.legislation.gov.uk/ukpga/2010/15/notes/division/3/6/1/2
Shockingly, the GRA2004 does not set a minimum age limit for diagnosis of "transsexualism":
GRA 2004:
25 Interpretation
“gender dysphoria” means the disorder variously referred to as gender dysphoria, gender identity disorder and transsexualism,
https://www.legislation.gov.uk/ukpga/2004/7/section/25
The GRA does however set a minimum age (18) for application for a GRC:
1 Applications
(1)A person of either gender who is aged at least 18 may make an application for a gender recognition certificate on the basis of—
(a)living in the other gender, or
(b)having changed gender under the law of a country or territory outside the United Kingdom.
+++++
This is how the concept of the "transgender child" aka "transsexual child" was smuggled into UK legislation.
It might be argued that the concept of "gender identity" as some sort of "inner essence" was also smuggled into the GRA 2004, in the reference to "gender identity disorder". However, even transactivist Law Professor Stephen Whittle and Lewis Turner could not settle in this 2007 mind-bendingly muddled article whether they thought the GRA2004 reflected a "new cultural concept of gender identity" or whether it was the same thing as "gender" or actually the determinant of "sex".
Whittle, S and Turner, L (2007) ”Sex changes’? Paradigm shifts in ’sex’ and ’gender’ following the gender recognition act?’. Sociological Research On-line, 12 (1). pp. 1-15. ISSN 1360-7804
https://e-space.mmu.ac.uk/621107/
Whittle was busy advising Parliamentarians on both the GRA2004 and relevant sections of the EA2010 so we have her to thank for the "gold plating" of both, going well beyond what was actually required of the UK Government.