Can't remember the poster name but ... generally schools (or other statutory services) do have to tell you they've made a safeguarding referral, unless by telling the family it will potentially put the child at more risk (e.g. FII or FGM). Also schools can't refer to Early Intervention services (such as 'Early Help') without parental consent.
I don't understand why so few can believe a school has ever made a malicious referral? Sure it's rare but it does happen. Schools are organisations like any other, made up of normal humans some of whom can be petty or vindictive. Referrals can be worded in a way that gets through normal checks and processes and sometimes teams of people can do unfair things - generally when there is a bad culture overall. Schools (like hospitals, police departments, local authorities etc) are not these magical places where everyone always makes fair and reasoned decisions.
Off rolling does also happen.
Anyway OP, my advice would be to just cooperate fully with Children’s Services while they investigate, do a SAR on the school and only communicate with them in writing from now on.
Personally, after the investigation I would actually deregister if you can afford it because I don't think these people are going to help you out or even offer evidence for the EHCP. You have time before GCSE years. However, if you don't want to do this for any reason, just hang in there, keep her on roll and continue to communicate with school in writing, e.g. 'Just to clarify, you are asking me to stay in school reception all day long so that you can provide my daughter with an education?