@Idontbelieveit14
Mine are allowed the odd one here and there. They are 13 and 14. They know when it’s appropriate and when it isn’t. They would never swear in front of teachers or their 4 year old brother, for example.
You might be surprised if you could eavesdrop upon them in school (especially when they aren't being directly supervised).
My usual approach is that there's a whole world of difference between swearing in front of a staff member (which can be a. entirely understandable in the circumstances, upset, in pain, etc or b. forgetting/not seeing that they're present, where a gentle 'could you try and think of alternative words, please?' usually gets you a variation upon the theme of 'Oh! Shit! Sorry Miss' 
) and swearing at a member of staff.
I do, however, remind them that whilst I can tell the difference in meaning, that doesn't mean other members of staff would feel the same way and alternative words would be a better choice for an easy life.
It's an important part of language and vocabulary development - just as they have to learn to use academic writing styles and adapt voices appropriate for creative or persuasive writing, they need to develop the appropriate language around friends, threats/creepy men/etc, people in authority ('Now, how far do you think you'd get if you spoke to a Police Officer/the Head Teacher like that?') and as an everyday thing 'Yes dear, I did hear you say 'Fuck, the fucking fucker's fucking fucked', but whilst I can tell you aren't particularly happy from your tone and facial expression, it's not actually clear what is wrong, so I'm going to need to know a little more detail'.
There's also the thing that some words are only deemed vulgar now whereas they were purely descriptive historically and some that were deeply offensive but now aren't. Applying modern sensibilities to those can mean that a lot of meaning is lost; but again, addressing those means another way of learning how to adapt appropriate voices for the particular circumstances in life.