An instrument is a search, not a category. Guitar, piano, drums and singing are looked for separately, and a page listing all of them competes properly for none.
Adults searching for lessons form a distinct market with hardly any competition. They ask different questions — how long until I can play something, does it matter that I cannot read music — and a page written for them converts unusually well.
Grade searches come from families already committed. Somebody looking up a specific grade is planning ahead, and a school that explains the requirement and the timing captures the whole preparation.
Enrolment has two waves: September and January. Both are predictable, both are searched weeks in advance, and both reward pages that already exist rather than pages built during the rush.