Set up the room with nine chairs in the middle of the play area in three rows or three. Split the class into two teams and have one team line up to the right of the chairs and the other team line up to the left, so that they are facing each other with the chairs in the middle. Have one group be the "O" team and have them practice making an "o" with their arms over their heads. Then have the other group be the "X" team and have them practice making an "X" with their arms. Then give each player on each team a number from 1 to how ever many kids there are. (The game works best with a group of twenty, and then each player on a team could have a number from 0-9).
Play starts when you call out a number, then the player on each team who has that number runs to a chair and makes their "X" and "O" symbol. Then continue to call out numbers until one team has a tic-tac-toe: where the "X's" or "O's" have three chairs in a row--Left to Right, Up and Down or Diagonal. When this happens that team gets a point and you start a new round.
The next level is to call out whatever math equations that equal one of the numbers that were given out. "1+2" ! (So the "3" on each team would then run on to the board.) or "Square root of 9" for higher level kids. From this point the variations are endless: Call out multiple numbers at once so kids have to race to chairs in groups, call out a series of equations, such as "1+5-2x3". If you are lucky enough to be playing with exactly 20 kids have them run as the full number, for example 12x2=24 so have number 2 and 4 go...
In The Moment:
Fast pace is key. They will want to discuss strategy,-- block or expand the row... There is also the issue of when a tic tac toe happens at the same time. It is the job of the caller to determine who arrived at the chair first, or declare it a tie. Also, keep kids at the same distance away from the chairs. They will creep forward. Some painters tape on the ground might be a good idea!
MATH: This is a very fun review of facts game that kids love to play. It doesn't really make the kids who have slower computation skills stand out because their teammates can't help by yelling out the answers and tell everyone to go. The beauty is that as kids get older the math gets more challenging, but the fun continues! I've had kids "solve for x", do square roots and exponents, you name it!
TEAMWORK/FUN: It is a great game to play with a group of kids when you have ten or fifteen minutes with them. You can deemphasize the math by just calling out numbers or making it much lower than their level. Math or no math, it is a really fun game that kids of all ages enjoy!
VOCABULARY: I have a fellow teacher that said they played the game with vocabulary words. Each player got a word, she called out definitions, synonyms, sentences with blanks where the word should go. I have not seen it happen or tried it myself yet, but I am told it worked great!