"This is from my dad’s senior prom in May 1971! [sic - 1970] This was taken at Little Cypress-Mauriceville High School in Orange, Texas.More info, including comments from Billy Gibbons, at Texas Monthly.
Apparently some time between signing the contract to play for the dance and the actual prom itself, the band broke out big. They tried to get out of the contract, but the school couldn’t find a replacement on such short notice so ZZ Top still performed. My dad said that people were climbing through the windows, crashing their prom, just to hear the band play. This was all at a really small school with a graduating class of around 100, maybe less."
Reposted from 2015 to add this photo of Private James Hendrix of the 101st Airborne (Fort Campbell, 1962):