It's really great that secondary schools are in a position to have psychology societies. I would have loved to join one when I was at school, as I really didn't know anything at all about psychology until I started undergrad.
If you do run one, what you may want to find out is if any of the parents of the students at your school work are psychologists (clinical, educational, forensic, research etc) and invite them to give a talk about their work. They could talk about their typical day, what they do, or how they got to where they are. There may also be people who work in the wider psychology workforce, like SENCOs, IAPT workers or psychiatric nurses, who use psychological skills as part of their work.
The British Psychological Society website also may be a good resource for you in terms of getting materials, accessing events and conferences and such.