If there is a training effect in increasing magical strength, and or improving concentration, keeping the rock transfigured works that way too, without being Transfiguration specific. Quirrel's armies help too - would normal classroom activity exhaust magic that much?

Harry could try a controlled experiment over the Easter holiday if he thinks about it. With McGonagall's permission, ask his classmates to keep a loop of cord transfigured into an unobtrusive platinum toe ring (or more than one) over the holiday when they're not otherwise allowed to use magic. Then compare deltas in transfiguration scores on the next test between those who did & didn't attempt it.

Is Harry getting letters back from his parents? He doesn't yet seem to be thinking about measuring magical strength, but a tool to compare would permit some kind of scale - if it could use embodied magic in magic items as markers. That could argue for faster access to later year charms. Similarly of pre-electronic tech that may be useful, that his parents may remember. (Slide rules, Curta calcs, Microfiche & hand held readers, ...)