Can a school/college (sixth form in Britain) punish you for smoking outside of school hours/grounds?

I sometimes smoke at a local park or just outside the train station about 300 feet from my school. It is out of sight of the school grounds, but teachers coming into school often come by there, and I see one teacher in particular a lot (my maths teacher, who I have brief conversations with when I see him, so he would therefore recognise me if he saw me smoking).
But I was just wondering, I am in the sixth form part of my school, it is a private school and quite strict on uniform and behaviour for the younger years because they represent the school.
But I am in the sixth form so I can wear my own clothes and to someone who didn’t know any better I would just look like a member of the general public, i.e. people seeing me smoking would not associate me with the school.
But would they be able to punish me if a teacher saw and recognised me smoking, outside of school hours, and not even in sight of the school?
I’m just wondering to what extent a school can get involved.
please do not lecture me on the dangers of smoking, I have been off the things for months but my exams are coming up and they are the only things that are stopping me from going completely nutso