Right, now we’re getting somewhere. Maybe not so much for a couple extra (though a fixed proportion of the funding will still be, per accounting rules, allocated to overheads). But don’t you see that you’re misunderstanding what “spare capacity” means? It means add a couple extra per class and the situation is probably favourable and the teacher’s workload and student experience isn’t too adversely affected. That’s your amount of “spare capacity”.
But once you’ve added a few more, the increased need for teaching provision plus the decline in teaching experience becomes a net negative. At a break-even point you then need to employ more teachers, and the costs start to outweigh the additional funding. That costs you even more in salaries, pensions, NI - more than the increased funding. When does that point get reached? Well, one school may be able to teach 30 maths students in a class with no decline in teaching quality or individual attention (though I doubt it). But in English, Art, drama, music or Chemistry? That tipping point gets reached earlier. Art and English teachers can’t increase their classes from 15 to 30 without reductions in their timetabling elsewhere. And students don’t want to go to sixth forms with 30 in a class. They start to choose elsewhere. So there’s a point at which adding more students creates diminished returns with the need for additional staffing, classroom space, loos, resources, admin, IT provision, etc. And all these pressures, despite what you think, are much more acute in the state sector, because of generally less good buildings, less staffing, more costs for pupil support, etc.
Spare capacity isn’t just merrily adding more bums on seats and assuming the increased funding is “profit”. Schools aren’t able to do that, either in their accounts, or in the effect on pupils. The idea that a private school exodus will be great for state schools just isn’t realistic. At a small margin, maybe. Not at 10-20 percent plus, which is what well may happen. We wait to see.