The most basic math Hangman game for 12-year-olds is simply a variation on the regular hangman game, in which you present a series of blanks standing for the letters of a word; students guess at the letters, and for each one they get wrong, they are penalized with another part of the hangman's body, until they have solved the word or the hangman is complete. Using a variety of math terms (from "factor" and "multiple" to "geometry" and "trigonometry") can help reinforce lessons from the classroom while providing a welcome diversion.
Complicate the Hangman game by providing students with a series of problems to solve, rather than words to guess. Here, you could provide pre-algebra problems involving variables; if students get the wrong answer when they solve the problem, they will earn another body part drawn onto their hangman. Working in teams, with consensus needed to propose an answer, can also help build classroom spirit.
Change the Hangman game into a form of connect-the-dots by reinforcing graphing that you have already taught your students. You can make a hangman (or a less morbid figure) by plotting a variety of points on a graph; give the points to your students and have them work to see which figure they come up with.
You can also return to the original version of Hangman by including math trivia, such as history and strange facts about math that your students do not know. One example may be, "This theorem helps students solve for the value of triangles: - - - - - - - - - - -." The answer, in this case, would be "Pythagorean."