Crossfit requires high levels of neuromuscular strength, endurance and memory. I’ve seen a dozen trainers or conditioning coaches put kids through crossfit workouts.
They often have a hard enough time doing a burpie/squat workout, let alone with weight and for speed.
I’d argue that for k-12 the kids need to learn the basics and build upon that.
For that age simply get them into starting strength programs- if a small number of them display great drive and proficiency then put them into crossfit.