One thing for sure, if the students who are currently in private schools have to move to State schools because their parents can't afford the 20 per cent increase, and those children can't get into the top state schools, then the let's say, average state school will see a massive increase in their GCSE Maths and English attainments and EBacc performance tables.
Also, private schools have the option of preparing students for iGCSEs which State schools can't, and some believe that iGCEs in some subjects are easier or harder or about the same. Those students changing over between Years 10-13 will be impacted the most. So I'm assuming parents will ensure, by possibly taking out a loan etc that their children get through their GCSEs and A levels, and those doing GCSEs, can easily get into a top State sixth form if they obtain the right grades.
With what's being done to the pensioners earning over £13k, I don't think there will be a lot of sympathy on this issue.