I have one lesson in which to teach 12 year olds about Hinduism. What is the single most important story I can teach them?

Hinduism in one lesson would be very difficult!

A condensed version of either the Mahabharta or the Ramayana would be good to teach them about Hinduism.

Each one has its merits.

Mahabharata:

Stand up for what's right even if it's against your family.

Only use violence when all other ways to solve the problem have failed.

Carrying out your Dharma.

Ramayana:

Only use the required amount of violence and no more.

Living for others and greater society rather than for your own petty needs like wealth, women etc.

Carrying out your Dharma.

I assume that's what you meant, but if you were looking for something else then email me if you want.

Meditation -- make them try meditation with you. We did that in my school.