DD's head of year (HOY) called me last year saying school was worried about DD's mental health. We then sorted it out at home over a holiday. However, after the holiday, HOY kept asking to see my child every day before school to make sure she was ok. 
She went along with it for a long time until a few months later. DD got into serious trouble in school and HOY made her see a counselor and made her see him basically every morning, break and lunch. It was all okay until I kept receiving calls from HOY every week asking how my child's home life was. Eventually, I stopped answering.
DD was friends with a teacher's child for a few years. When the teacher found out DD was in trouble, the teacher told the child that they would be in trouble if they contacted my child again. 
She didn't know why her friend was avoiding her so she contacted her friend. The friend replied saying they never wanted to see DD again because their parent was worried about rumors that teachers had alerted the parent about. DD went to teacher asking why they told the friend. The teacher said students had gone to them about the rumors and that DD shouldn't tell them how to 'care' for their child. HOY then called the next day and brought DD to the office to shout at her about the incident.
In addition, on 2 separate occasions, almost £1,000 of various items were stolen from DD's bag and blazer while she was out doing P.E. She alerted teachers and they said they would check security cameras outside to see who was going in and out. No further action was taken about it despite even more objects getting taken on later dates.
DD is quite short and wore a 16 inch length skirt at school. Other students are a few inches taller and all wear shorter skirts about 14 inches when measured in class. She then brought a longer skirt by a few inches. In the first few days of school this academic year, a senior staff member would comment on her skirt length saying she wanted to see 'less of her,' that she needed to 'buy a longer skirt,' that her skirt was the 'shortest she'd seen all morning' and that she would 'have to wear tights' if she wanted to continue wearing that skirt. Also, the same teacher said her 'tie was too low it was invisible,' criticizing her uniform every day. She was with friends when this happened, taller friends with shorter skirts! 
Months later, there was still nothing about my daughter's makeup.
DD was still seeing HOY every morning and getting shouted at for no reason. Her best friend (BF) was also seeing HOY and she got on well with HOY. BF was never shouted at and was always let off when she got into trouble. She asked HOY why they had it 'in' for DD and HOY gave a sarky remark which was quite rude. BF told DD. DD thought nothing of it .
DD went to see HOY with some issues she was having and HOY blamed it all on her, saying everything is her fault and shoved her out of the office as soon as possible. A few days later, she returned to HOY with the same issue and they shooed DD out of the office. DD never got on with any teacher terribly well (apart from her ex-friend's parent teacher thing mentioned before). She went to another teacher to talk but they didn't help either. That night, DD felt sad and ended up in A&E which was not a nice experience. 
Teacher's didn't help DD but they help her BF. HOY hated her but loved her BF. Ex-friend's parent teacher thing lied to separate DD and her friend. Senior staff target DD about her uniform. Teacher's do nothing about the £1,000 stolen from my child.
I can't help but think my child is being targeted by the staff at school. She can't move school due to there being no spaces and also due to exams etc. I don't know what to do. I would sue to the school but I have no solid evidence (apart from her school friends that witnessed the events) and lawyers are expensive these days! 
What can I do?