Ive done an apprenticeship course, not in window cleaning but the idea is that the company gets you to work for cheap and you go to college maybe 1 day a week to learn something relevant to window cleaning or whatever your doing, you could probs send them to college for some kind of trade like plastering or something and say its a property maintenance apprenticeship. This is how I see companies do it all the time, when I was 16 earning £100 a week on £2.60 per hour that was loads for me.