Hi, I would please ask you which of the expressions in bold is correct in the following (I used former because my friend and I are not at high school anymore):
I have a former high-school friend who is working there.
Maybe you are wondering whether "former" is ambiguous and could imply "former friend"—that is, that you are no longer friends.
I would probably say "I have a friend from high school who..."
But this graph shows that it is not wrong to say "a former high school friend".
It is logical that the phrase "high school", when used as an adjective, would be hyphenated;
but in practice it is usually two separate words, as shown in this graph.