Steinberg’s educational FAQ might help, though it doesn’t really cover this scenario that well. Unless you get a definitive answer from a member of Steinberg staff, I suggest they apply anyway. A student (or, in this case, I suppose, a student’s parent or guardian) adds an educational product to their cart, then when you get part way through the online checkout process you are asked to upload proof of entitlement. This happens before you part with any money - if the educational request succeeds you are able to log back in and check out. When I did this recently, my purchase was authorised within a couple of hours.
I can’t see why a junior high or high school student should not qualify - presumably they have some sort of proof of enrolment that they can scan and upload. If not, presumably you can provide a letter on school headed paper confirming they are your students.
The one thing that is clear from the educational FAQ is that Steinberg’s educational scheme goes far beyond those in higher education.