You conflate, again, two things. Government policy, which is that masks should be worn but are not legally required to be, and how a school may choose to implement that policy through inclusion of masks as part of uniform. The key word here is “may”; different schools will I’m sure apply different approaches.
According to government, school uniforms are entirely at the discretion of schools, yet those policies form a basis for school discipline. That, unless you can point me to legally binding guidance specifically barring a school from excluding a pupil not complying with school instructions on mask wearing (presuming of course that the breach meets the requirements of that policy for such sanction), leaves schools with the option if they include masks in their uniform policy.
In the autumn, my children were at schools that included mask wearing instructions within the uniform policies and did therefore have the right to impose sanctions (including requiring a pupil to return home to obtain compliant clothing) for breaking mask wearing rules. I fully expect the same to apply this term, and in practice for compliance at both schools to be good enough that overt enforcement is unnecessary.
As
@nedchester suggests, I am a serving governor but not at either of the schools my children attend.