Split the students into two lines however you wish, this can be boys v girls, young v old or simply those who sit on the right side of the classroom v those who sit on the left.

The students at the front of the line have one chance to listen to the word you say and correctly say the opposite word before the opposing student can do so. If they are correct then they can sit back down at their desk while if they are wrong or too slow then they have to return to the back of their respective team's line.

Continue this process until one of the teams has everyone sitting back down at their desk, they are the winners! Make sure the other waiting students remain quiet to keep the game fair and feel free to play more than one round to give the losing team a chance to improve. You can even introduce a points system where the winning team gets an amount of points equal to the number of students left in the other team's line.

Here are some examples of the opposites (antonyms) I use:

fast - slow
happy - sad
heavy - light
good - bad
high - low
hot - cold
in - out
north - south
on - off
new - old
left - right
tall - short
true - false
east - west
yes - no
up - down