I think newcomers, who must get acquainted with the language and environment, should benefit from a system that doesn’t get too much in their way by default (as opposed to having to handle warnings using whichever strange incantation) while they’re learning, and keeps displaying the same relevant data even when rebuilding.
In fact, when teaching, as programming projects and lab sessions remain quite small in LoC and number of files (< 10), we don’t need dune’s fast compilation but out-of-the-box: showing (without failing) warnings at each rebuild, fast dune utop, one short configuration file with almost no clauses by default (declaring the libs to use should be enough), easy testing (thanks to ppx_inline_test), no upwards exploration… We still want to use dune as it’s the most standard tool nowadays, supposing than we can charm a couple of students into keeping programming in OCaml even after the end of the semester 