They are covering the costs of training, employing and then replacing you should you leave within 3 years. As you might well be tempted to. You didn't say whether it is a reducing bond pro-rata over the term. That might produce some more useful guidance for you.
Schools cannot afford to invest money and lose their staff in the same way as airlines. Yes it may not appear to be 'as much' overall when compared to airline opportunities, but then there is much less profit margin and revenue when you consider schools v airlines. Apples and Oranges.
If you are committed to teaching and someone is prepared to pay for it (including valuable IRI experience), three years doesn't seem too much to ask in return. The other way of saying this is: I don't think its about the costs of training you, or reparation if you leave - its about how to monetise a proposition where the FTO is essentially saying: We want you for three years. Yes or No?
Interested in how this one will run...