"This week, contractors removing old chalkboards at Emerson High School in Oklahoma City made a startling discovery: Underneath them rested another set of chalk

"This week, contractors removing old chalkboards at Emerson High School in Oklahoma City made a startling discovery: Underneath them rested another set of chalkboards, untouched since 1917." So, presumably, the "old chalkboards" being removed by contractors have been there since 1917, or approximately 100 years.