I would start by trying to find out as much as you can from your daughter - it seems as if she is going in to school, as you dropped her off. What would make a difference? Somewhere to go at break times? Mediation with friends? Change of teachers? TA support?
Then make appointment with head of year, and get their side. It may be that your daughter is happy in some lessons. It would be useful for you to know how they see the problem.
Keep a paper trail - whenever you have a meeting, you'll probably get an email from the school. Challenge anything incorrect. If you don't get an email, send one confirming everything that is discussed. If you don't get anywhere with the HoY, go to the Head.
The starting point is to find out what makes your daughter anxious - then you know what you would want the school to do. If your daughter can't tell you, or just says everything, it's harder, but still keep communicating with school.
If you can keep getting her into school, that's great. Avoid absences as much as you can because the more she's absent, the harder it is to go back in. If she can't go in, don't let it go on too long before asking for work to be sent home.
If you suspect a medical issue, get advice from your GP. If you think it's a learning problem, ask for assessments.
It's quite likely that school won't be able to do much - there just isn't the money or staffing, but you want to make it clear to them that you want an education for your daughter, and you want to work with them.
Hope that ramble helps!