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.