Depends on what they’re marking. Opening of the school? First graduating class? If the first school year was 1922–1923, then the class of ‘23 would mark the 100th anniversary of the first graduating class. If this were my problem to solve, I would ask what the actual date of founding was, not just the year, and then observe it in whichever school year it happens to fall in. Or both classes could celebrate. Why not? They could also celebrate the 94th (or 95th) right now.