She sounds overwhelmed, school can be incredibly hard for our kids - so much stimulation, structure and so many expectations about behaviour and that’s before you get to the learning part. Starting school can really highlight gaps in social, emotional and cognitive development that weren’t apparent previously.
Firstly, trying to impose control is a recipe for disaster, the very nature of dysregulation means she doesn’t have control in the moment, so you trying to impose control will set you on a collision course with her. I’d be working with the school exploring what her day is like, are there regular trigger points for her? Kids with an early trauma history often find transitions very difficult so think about arriving at school, moving into the class room, settling down to work, transitions to break time/lunch time etc. Visual timetables and now, next and then boards can be really helpful because they remove uncertainty which in turn reduces overall stress which is what you want to do for her. What understanding does the school have of the impact of developmental trauma on children, if they have a decent understanding it’ll help because they’ll be open to working with you.
Think too about your pre and post school routine, try to build in time for her to decompress and don’t rush off to other activities, shopping etc. We came straight home from school to a snack (milk and banana, not sugar) for years, some colouring or quiet play or story time for years. Also look at her sleep, my DS needed a clear 12 hours sleep in primary 1, sometimes by the end of the week he was falling asleep into his dinner. While home routine might not seem related to school, it’s all interlinked, if you reduce uncertainty and create solid routine and keep her world outside of school quite small, she’ll have more capacity to cope with the demands of school, fears about friends etc.
Can she tell you how she’s feeling, can you use story books or social stories to help her make sense of how she feels in the school day? Anything you can do that helps emotional literacy will also help her self regulate, so talking about how you’re feeling (tired, worried, sad etc) will help model that as will looking at story books and helping her think about how the characters in the book might be feeling and asking her if she’s ever felt that way.
There are so many things you can do to support her, but look across the whole of her week, not just what’s happening at school.