AIST hires qualified mathematics and physics teachers for every programme. Every other role, from tutoring to marking to organising, is filled by volunteers, and almost all of them came through the summer school as learners.
Tutors join in their second year of university or later, matched to programmes by what they study, and deliver the astronomy component themselves. Learners still in Grade 12 or first year take on support roles instead, so that nobody loses contact with the organisation while they settle into university.
This is also why the programmes run in June and December. The people who staff them are university students, and those are the months they are free.