Teach them that without context, you don't know and that to ensure they are clear when giving sizes in bytes themselves they should use the IEC prefixes instead of SI ones: 10 kB is ambiguous, 10 KiB is not. Also, do this consistently yourself.

Teach them that without context, you don't know because there most certainly are people out there who will use k to mean 1000 and others who will use k to mean 1024. Which is right is not relevant because both usages are out there. This leaves any use of "k" with bytes ambiguous unless whoever gave the number also specified what they meant.

For this reason I'd recommend that you teach that when giving a value in bytes, always use an IEC prefix like Ki instead. 10 kB is ambiguous, 10 KiB is not.

We can declare certain usages are "wrong" all we want, and I'm not saying that is necessarily unjustified, but that doesn't make those usages go away.