A. Taking time out to work for a charity seems like a good thing to do. As far as the school is concerned, this should not cost them any more money than the expense of advertising for your replacement, assuming nobody in the school can take over. As you teach French, finding an external replacement should be relatively easy, although the fact that you teach in a middle school might make it slightly more difficult than for a more typical secondary.
How do you persuade the governors, or, to put it another way, what's in it for them? This depends upon where you are going and what you will be doing.
Working in a school in a Francophone country in Africa might mean you could set up email exchanges with your school. You could help your pupils understand about developing countries and extend their knowledge of the French language spoken in that part of the world.
There would also be possibilities for geography, science and other curriculum work. Not only would it be exciting, but it could be motivational for pupils in both schools. However, if you are aiming to work for a charity outside education, you might need to think of other reasons for the governors to be helpful, based upon what the experience of your time abroad might allow you to bring to teaching in your particular school.
As for your pension, you will probably miss a year's worth of contributions, but you can investigate whether it is possible to make additional payments when you return to teaching in order to bridge the gap.
Your last sentence suggests you have made the decision to go, whatever the governors say. There is always a risk associated with this type of move and only you can weigh the personal benefits against the risks. But the extra experience should help you gain a job when you return John Howson is a recruitment analyst and visiting professor of education at Oxford Brookes university. To ask him a question, email him at email@example.com