Is it legal to add a teacher on Facebook once you have left school?

A1iice
So I know this is a little random, but I am curious. I would like to add my teacher on Facebook, I have currently left school so I am no longer her student, I would of thought it'd be OK, but then I am unsure, I don't want to get either of us in trouble.
Is it legal to add a teacher on Facebook once you have left school?
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