English made his comments after a front-page story in the Herald about new guidelines from the secondary teachers' union.
The Post Primary Teachers' Association (PPTA), says both boys and girls should be able to "choose from a range of shorts, trousers, skirts of different lengths and styles, with both tailored and non-tailored interchangeable shirts".
"It is important that access to specific uniform items is not limited on the basis of biological sex or perceived gender identity," the guidelines say.
They also urge schools to provide "individual toilet and shower units with lockable doors and floor-to-ceiling divisions" and "options for students to change and shower in privacy".
InsideOUT co-chair Kate Aschoff and her friends campaigned successfully last year for Wellington East Girls' College to introduce shorts as a uniform option.
Aschoff, now 18, said her trans friends had to battle to wear pants, get their teachers to use the correct pronouns or have their name changed on report cards.
Proactively offering other uniform options and gender-neutral toilets could cut the drama out of what can already be an uncomfortable situation for a young trans person, she said.
The issue has become politically sensitive since President Trump reversed an Obama Administration measure that gave transgender students a right to use the bathrooms of their choice.
Avondale College biology teacher Shawn Cooper, who led the PPTA's Rainbow Taskforce which drew up the guidelines, said wearing skirts often put girls off physical activity.
Auckland University's Youth 2012 survey of secondary school students found that 68.5 per cent of boys, but only 56.5 per cent of girls, did more than 20 minutes of "vigorous activity" at least three times in the past week.