KINGSPORT — It’s almost time for Kingsport to hit the books. Students from Kingsport Theatre Guild’s Teen Camp will teach history, grammar and math through clever and tuneful songs in their production of “Schoolhouse Rock Live! JR.” Friday evening. “Many of us remember ‘Schoolhouse Rock’ as the Emmy Award-winning, three-minute Saturday morning cartoon series that taught children how to unpack their adjectives, memorize the Preamble to the Constitution, and discover that ‘Three is a Magic Number,’ ” said Drew Cohen, president and CEO of Music Theatre International.