It really doesn't 😂 OP they're school horses. They've been specifically trained to ignore the rider unless it is a very clear and decisive cue.
Horses aren't push button. They pay attention to everything. Your leg, the reins, the minutest shift of your weight, it all means something.
Then you have the learner. The learner does not sit quietly.
They wobble, they fidget, they have no idea how to balance without moving their legs, they hold their reins unevenly, flapping about like washing lines, while also using them for balance. They are basically asking the pony to dance the Macarena backwards, while simultaneously telling him to hold still and also please launch into canter from a complete standstill.
These ponies know to ignore it all. If they aren't 100% certain that was a deliberate kick to move forwards, they will wait for another one, just to be sure. For example, at our school we have a little boy who shifts his weight backwards and kicks at the same time. Pony is confused. Sit back means stop/slow down and kick means go. So pony waits for clarification. This is frustrating for the child but learning to control your body is all part of learning to ride.
Girthiness is also common. We have several like that. One is due to having previously had ulcers. He kicks back and bites because he remembers that it used to hurt. He still expects the pain. He is so stoic, no-one knew he had them until he started showing symptoms and by then he was in major pain. He's been girthy ever since.
One us because one of the kids accidentally pinched his skin when doing it up once and now doesn't trust that it won't pinch him again.
The other is just grumpy in general and wants to be left to eat until he can't fit through the gate.
Then we have the fact that horses have moods. One hates the rain and bites anyone who has the audacity to exist anywhere near him when it's raining. Sometimes they're just grumpy.
If she wants to be around horses, she needs to get used to the fact that she will get bitten, stepped on, kicked at, barged into the odd wall, she will fall off, sometimes get bucked off and sometimes it will hurt, sometimes it won't but she will get up and go again. Horse girls have to bounce.