I can see making a thread of "Unlikely High School Gym Performers that Freepers have seen". For me it would be José Feliciano (1966?).
Re Lovin' Spoonful, back then I memorized the lyrics to some of their songs. These days, the lines for "Old Folks" sometimes surface from those depths!
The Lovin’ Spoonful and the Strawberry Alarm Clock concerts were put on by the Willimantic State Teacher’s College ( now Eastern CT State Univ. ) . Back in 1967/8 they didn’t have a large enough building to hold the concerts so they borrowed our gymnasium .
It wasn't a high school gym, but I went to the Grand Opening of the Park Shops Mall (aka The Shops in Houston Center, aka The Highlight at Houston Center) when it first opened in early 1983.
Their featured musical act was The Association. They played all their hits (Windy, Cherish, Along Comes Mary, Never My Love).
-PJ