Class Plates

Well...a few days back a few people informed Radha Ma'am that the Director had seen some students from class XII swapping the class plates of the Computer Lab and the nearby Girls Toilet.
Well...everyone knows who the students are...(we'll let them get away this time...because of what follows).
Radha Ma'am informed us about the incident only to realise that it brought loads of laughs on the face of every student present.

Then she told us to stop doing such stupid things ( she's right BTW...so u 2...please stop.) because the teacher's aren't so stupid enough to realise the change. Well...is that so?
Last Year, a Student from Class XII-B [Name withheld to protect privacy ] switched the plates of Class XI-A and XI-B at the end of the year. And nobody noticed it. Yes, that means that the current XI-A was actually XI-B last year and XI-B was XI-A before.
What happened to the teachers then? Looks like the change went unnoticed. Maybe they thought that the change was intended. But then even the Principal didn't notice the difference.
Sort of Ironic isn't it?