Three resources must coincide. A boat of the right type, an instructor with the right qualification and enough students to make the week viable: any one missing and the course cannot run, which is a decision better made three weeks out than three days.
Certificates are records with consequences. Who was assessed, by whom, on what date and to what level has to be issued correctly and retrievable years later when somebody wants to charter abroad.
Bad weather moves whole courses. Rescheduling five days for eight people, several of whom booked flights, is not the same problem as postponing an afternoon, and it needs handling as a single decision.
A cancelled week cannot be refilled. Deposits and a written cancellation policy are what keep a school solvent when somebody withdraws in June from a course booked in January.