I find that a lot of students use the ordinary hyphen-when they
intend a break that is parenthetical in scale-as I have done in this
sentence.

What is a "hyphen-when" I ask.

This is a particularly hard error to correct in a marginal note.  You
practically have to write them a letter every time it happens.

Personally, I prefer -- and use -- two little hyphens with a space
before and after.

R.E.