Girls are excluded from sport or activities on the basis of gender alone
This argument relies on the historically constructed idea that certain activities are inherently gendered, and uses that tradition to exclude individuals who do not conform to it. It commits the is-ought fallacy — because something has traditionally been one way does not mean it ought to continue that way, especially when a constitutional right is at stake. Assigning activities as 'for boys' or 'for girls' is a form of gender stereotyping that the law explicitly prohibits.
They say: 'We have never had girls on this team and we are not starting now.' You respond by: Citing the constitutional prohibition on gender discrimination and the Schools Act's non-discrimination requirement, without aggression.
After you give your response, they may push back. Here is how to handle each counter-argument.