Two X

[Sajan]

Recently I've come across one phrase that puzzles me. It's "Two X".

Here's the extract:

...they were a party from a school that had not broken up yet. They all wore blue uniform blazers, and they were followed by a teacher who was shouting even louder than they were.
“You are to walk, not run, Two X!” the teacher screamed.
She was the kind of teacher nobody listens to.

Is it sth about school grades? What exactly "X" stands for and why is here "two"?
I'd appreciate your help. =)

[Mister Micawber]

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I suppose it means 'two by two', that is, in pairs.
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_________________
Native English teacher at Mister Micawber's

[Sajan]

Oh. Is that so? I could never guess. Thank u very much. =)
So "X" may mean "2"? I wonder where that came from... ^^;