@primaryproblems girls don't present the way that boys do (in general) with SEN. I can tell you now that your DD's teacher has no idea what she is looking for when it comes to SEN. Teachers, generally, get very little teaching on SEN and the ones they notice as having SEN are going to be the ones that cause them difficulty: Difficulty in behaviour management, difficulty in assessing them, or difficulty in catering for their needs. They will not notice the compliant, quiet, passive child as having SEN.
DD2 was just like your DD. Reception was 'ok' because it was just play and she could opt out of anything she didn't like, avoid mingling with other kids she couldn't bear, and make her repeating patterns. Year 1 was a completely different ball game, and the things that other kids took for granted, really bothered her. The tables were in a different direction. The chairs were different colours. They wanted her to use cursive writing instead of print. The children wanted to play actual games at playtime. It was a long day. She was exhausted. DD2 actually became physically ill because of her school experience, but actually, it was burnout.
I could write a book about how we've got to where we've got, and I won't bore you with it, but DD2 was diagnosed with ASD in year 7, when she was just 11, having been referred at 9 years old. Safe to say that by year 10, DD2 was so burnt out from trying to cope with school that she broke. If it wasn't for the COVID lockdowns, she probably would have broken in year 8, but she was at home at that time.
Now, DD2 is repeating year 11, but hasn't been able to cope in a school environment. The LA tried giving her an independent special school place, but she couldn't cope with the behaviours of the children there. She now has an EOTAS package from the LA.
You need to listen to your child. She is telling you that she can't cope. 6 year olds don't do that. I will bet my bottom dollar that your DD is neurodiverse and is just very bright, so is masking at school.
Incidentally, my DD3 is going through the same process now that she is in year 10. She broke last year and we're just going through the EHCP process for her.