The only way this would make economic sense would be quite immoral from my point of view, so honest question: why should an organisation pay them such salaries

The only way this would make economic sense would be quite immoral from my point of view, so honest question: why should an organisation pay them such salaries, unless they can effectively enslave the taught pupils until they've made that money back? Or are you arguing that it should be done through socialism and taxes? In that case, whats the rationale for that? The society won't gain significant value from this, and would have significantly less money available for other things. Do keep in mind that teachers usually still get more money then the average salary while having significantly more time off-work.