Two teams are formed by evenly dividing a number of players.
Each team lines up against the back wall of a gym/stadium/collosium.
A line of balls are placed along a line that divides the arena.
At the sound of a whistle, the teams charge the center. No team member is allowed to cross the center line.
Each team savagely hurls balls at one another. If you get hit, you are in the opposing team's jail. If you catch an opponent's hurled ball, that opponent is in your jail.
The only way to get out of jail is to catch a ball thrown by one of your teammates. The opposing team's jail is situated at their back wall, making a "jailbrake" difficult. The opposing team is allowed to "guard" the jail.
A team wins when all of the other teams members are in jail.
The best way to get your gym teacher to let you play jailball is to fit it into random sentences. Example:
"Hey teacher, the JAILBALL sure is nice today."