you don't say how old she is; if young, she may have had a rough time when backed & started, so associates the school with bad memories. (that can happen at any age!)

Have her back & saddle fit checked immediately, she could be getting pinched; my mare's very sensitive & was very one sided to start with due to injury as a foal & the saddle pinched into her right shoulder. This would show up more on circles than on the straight.

TOP TIP

use your mobile phone camera to take pics of her back from above after riding without a numnah or anything else under the saddle to see if it sits straight or to the left or right - you could sprinkle talc on her back first to show up any pressure points.

also as you ride, just slide your fingers under the front of the panels down by her shoulders to see if it feels comfortable for you, if it hurts you, it will be hurting her more!

try using a wider softer girth possibly with a central elastic insert NOT elastic on one end & a wool girth sleeve.

I would go back to doing groundwork in the school for a while building up your bond with her & gaining her trust so she looks to you for comfort when she's scared.

I would not ride her in the school for some time until she's a lot more relaxed about being in there with you on the ground first as you're not achieving anything at the moment & IT'S POINTLESS EITHER OF YOU GETTING INJURED - do the groundwork, gain her trust & make it fun for her.

In my view, the most important thing to do is the moment she makes a move to do what you ask, you must be over the top with praise - we all like being told how clever we are & horses are just the same!

i hope this is helpful to you, good luck