@Mrsbabbecho ,
Have you looked at the model?
The upside is the VAT collected plus the additional VAT collected on the additional vatable expenditure from the freed up income from pupils who move. The downside is, obviously, the additional cost to educate the moving pupils.
Now, everyone on this thread will say that when they save £20,000 a year, they won’t spend any of this on vatable items but either increase pension or stop working (or work less), but we all know, in reality, that is just written out of spite and is, colloquially speaking, a load of bollocks. (Don’t bother saying how this will really happen. We will see real numbers soon enough).
The IFS modelled all the above and, depending on the amount who leave and the propensity to spend, say it will raise money.
Now others (many with a vested interest) have modelled it differently and say the IFS is wrong.
So far, few have left and the IFS is looking right, but it is a live experiment, so we will soon see.