planning, marking, resourcing (those sheets of paper won't laminate themselves!)
reviewing schemes of work, where you're up to, working out what you have left to teach vs. how many teaching hours vs. the need to do some revision
pastoral care of tutor groups - dealing with anything and everything that comes up from friendship group issues, teen anxiety, more serious safeguarding issues which need to be passed on etc.
endless data input, doing stuff to generate the data to input (assessments, mainly, which need setting and marking)
generating publicity material (private school) which means whizz bang activities that photograph well
lunch time catch up lessons for exam classes
lunch time catch ups for students who are not in exam classes
after school lessons for exam classes
running extra-curricula activities and competitions (see publicity, above)
jumping through hoops to make books 'pass' book scrutiny
supervising detention (on a rota)
duty at break/lunchtime/afterschool (standing around in prominent places to watch what is going on)
revising or producing materials sent to parents on exams, homework, options etc.
calming down a particularly highly-strung colleague
attending briefings
reading and responding to emails
calling or emailing parents about students either from an academic or a pastoral perspective
engaging in CPD activities (some of which are 'enforced', some of which are voluntary - so keeping up with changes to exams is particularly time-consuming at the moment!), I read to keep my 'A' level subject knowledge current, review social media groups for teaching ideas (and shortcuts), read subject-relevant blogs of respected contributors etc. etc.