This kind of a plan sounds great, but is IMO close to untenable in the real world.

Build a great big school.

Out of what ? Sure, you can build the building itself. But you also need (among other things) electric power, a reliable food supply, clean water, medical care, computers, plus a ton of muscle to protect you from people who will want to take all of the previously mentioned stuff. Poor countries have none of that. Well, they might have some muscle, but reliable security is tough to buy.

Offer the following deal to parents of gifted children: they send their children to you, and you'll educate them for free, for ten years

You will be overwhelmed with offers in a matter of days. How do you decide which children are gifted ?

If at any point they do start working, you get (say) 10% of their income for 10 years.

How will you enforce that ? Actually, before you can enforce anything, where will your graduates find work ?

Obviously you still need some instructors.

Where will you get them ? Do your kids speak English ? Do your instructors ?

Do it morally

Trust me, this will be the least of your worries.