You will live with this decision
for the life of your school.
The classroom structure you set up in the first hour determines how you navigate your school for years. A structure built around current teachers becomes fragile when teachers change. A structure built around specific cohorts requires rebuilding every academic year. The right structure reflects permanent features of your school's operation — subjects and year groups — rather than transient ones like current teachers or current cohorts.
Subject → Year level → Classroom.
Then teachers and students.
Classroom (year group within subject) — ‘Year 9 English’, ‘Year 10 English’. Stable over time. When a student moves from Year 9 to Year 10, move them to the new classroom. The old one gets archived at year end.
Enrolment (students in a classroom this term) — The transient layer. When a term ends, archive the enrolment and create a new one.