Sex is a protected characteristic in the Equality Act.
Harrassment is a type of discriminatory behaviour that is unlawful under the EA:
(1)A person (A) harasses another (B) if—
(a)A engages in unwanted conduct related to a relevant protected characteristic, and
(b)the conduct has the purpose or effect of—
(i)violating B's dignity, or
(ii)creating an intimidating, hostile, degrading, humiliating or offensive environment for B.
I think a decent case could be made by parents of girls that this head teacher is harrassing female pupils on the grounds of their sex by bringing in drag queens for compulsory lessons.
Unless it's a private school they'll also be required to comply with the public sector equality duty which means they not only have to not discriminate, they have to actively consider how the decisions they make affect different disadvantaged groups and how they can actively advance equality.
Everything Datun has said about grooming is spot on.
Beauty and Misogyny is an excellent book and you can download it for free here.
While looking for that, I noticed Unpacking Queer Politics is also available here. Everybody should read this book!