Honestly all gender stereotyping is bullshit. But being the kid to fight it depends on the kid.
Both mine, having been given free choice as toddlers, wanted to continue to mix and match their styles in primary school. And both (different schools) got the oh so predictable: "you can't wear THAT! You're a boy!" and the new progressive version: " you must be a girl now, because you're wearing purple/ gold/long hair etc"
One kid found it upsetting and has gradually brought his tastes in line with the junior gender police. And the other (more self confident) shrugged and continued to present himself how he fancied.
Bullying should always be reported obvs but not all kids are saying this stuff to bully. They're simply pointing out what they know about the sexist society they find themselves in.
Re those school shorts though.... If you get the ones with combat style pockets on the leg kids love them because they can fill them with found treasures.