I remember learning in school about how men in England were grouped together in a group of 12 or so and were responsible for eachothers actions (breaking law, debt, etc). What was the name of this practice?
Is it possible that you're referring to the Frankpledge?
That wasn't a group of 12 men, but it was a group of men over the age of 12. Their households were collectively responsible for one another's conduct.
For completeness, both @RazieMah & @andy256 point out that the term you're seeking may be tithing, which is a territorial unit (1/10th of a hundred) of people who swear the frankpledge. Credit to them, but I'm updating the answer so that it is a better reference.