A single desk height cannot serve a Class 1 student and a Class 10 student, and yet that is exactly what most schools buy. Ergonomics for children is not one problem — it is five, one for each age band, and getting it wrong shows up as slouching, fidgeting and back complaints long before anyone connects it to the furniture.
The governing numbers are seat height and the seat-to-desk differential. For a 6-year-old, a 26cm seat height with a 20cm differential keeps feet flat and forearms level. By age 14 those numbers climb to roughly 42cm and 27cm. Between them sit three more graded steps.
We build our school ranges to these bands rather than a single "medium" size, and we colour-code the frames so a teacher can tell at a glance whether a desk belongs in their room. It costs nothing extra to specify the right band — it just requires asking the question before the order goes in.