Yep, sorry, I really don't get this at all. 
I know it's often possible to be in a real-life situation where you are absolutely certain that there was sexism in play, and to find when you recount it, other people can't see what you saw and come out with well-meaning 'but are you sure it was because you were a woman?' comments.
And I think we should all be a damn sight better at saying 'erm, yeah. I was there. I am very sure it was sexism.'
But I don't think this article communicates any of that, and it comes across as really sneery to the noisy students, despite the careful insistence that you weren't sneering. Sorry, but it does.
I once had a guard on the train tell me he wouldn't make a bloke turn his phone off 'because he's working, love' (this, while I had my laptop out and a thick pile of papers, which just might have suggested I was working too!). So I know people can be sexist.
I just think you didn't really communicate why on earth you think this was sexism and why you didn't ask them to be quiet yourself.