Every slot is perishable. A one-to-one lesson at five on a Tuesday that nobody takes is not recovered later, so knowing which slots are habitually empty is the difference between a full timetable and a fortnight of gaps.
Teachers and rooms are separate limits. Four teachers and three rooms produce a timetable that looks possible on paper and collapses on Thursday afternoon unless both are booked together.
Instruments on loan need a record. Who has which one, since when and in what condition: without it, a school discovers at the end of the year how many never came back.
The rescheduling policy is what holds the week together. If a missed lesson can always be moved, the timetable becomes fiction — a written rule, applied consistently, is what protects everyone's time.