I don't know where the child was previously, but if she went to the private school in Y7, that move would have been done in Autumn 2021, so before Boris was ousted, and with the Conservatives sitting on the sort of majority that most would have expected another win at the next GE.
The vast majority of people were expecting the last GE to be Nov 2024 - Jan 2025 even if it was clear the Conservatives were unlikely to win. Nobody saw July 24 GE coming.
So, parents could have reasonably assumed that Labour would take a year to implement the policy and that it would be introduced from the start of an academic year.
This would mean it coming in for either Autumn 2025, or even Autumn 2026 (given the 2 years foreseen for vapes that doesn't seem unreasonable for something this complex). By this stage, the child would be going into Y11 and so the parents would only need to find the VAT for a year.
There was also the possibility that VAT might have been introduced for those starting YR, Y7 and Y12 rather than all year groups.
The child's parents have been rather sensible to realise that they can't afford 3 years of VAT and jump ship now in Y9 rather than with potentially greater numbers for the start of Y10. Or finding themselves in a real mess and unable to fund school at the beginning of Y11.