cross mark; X mark; demerit mark
Refers to an X-shaped mark, often used to indicate something is wrong, incorrect, or unacceptable, such as on a test or form. Can also refer to a demerit or penalty point in a scoring system.
The questions I got wrong on the test had cross marks on them.
You get one demerit mark if you're late.
The teacher marked the mistakes on the test with an X.
The wrong answer got an X mark.
If you're late three times, you get a demerit mark.