You can join a match in two ways.
Current live games: these are games started by your peers from your class
Join with activity code: join any game by entering its activity code
When you join a game, you'll be in the lobby until the host starts the game.
If the host aborts the game in the meantime, you'll see this screen.
You can also exit a game after the host has started it.
You'll see this screen if there are other players yet to finish the game.
The scoreboard will show your score and the top three scores.