End the game FOR EVERYONE, and return final scores.

Until now the only control wired to this lived on the final scoreboard — a screen you reach by playing the game to its natural conclusion — so a game in progress could not be stopped at all. It is reachable mid-game now, which makes the question of who may press it a real one: see engine.may_end_game. The teacher, or whoever started it. Everyone else has the header X, which closes the panel for them and leaves the game running.

The scores stand. They were earned, and ending early is not a reason to throw them away.