On Sumdog you can choose to play your classmates or other Sumdog students.
Log in to Sumdog and click the green button on the bottom left of the screen. Here you can choose to play World, Class or Robot.
If you play world, you'll play against other Sumdog students.
Playing class lets your play against other students in your class at school.
Playing robot will match you with a computer opponent.
Playing the robot and world only works when playing Sumdog's Maths and Reading games. When playing Sumdog's Writing games, you'll only be able to play against your class or family.