NEW YORK (MarketWatch) -- Investor Whitney Tilson of Kase Capital said his largest short position is a bet against technology-based education company K12 Inc. /zigman2/quotes/205813628/composite LRN -3.85% . Tilson, speaking at the Value Investing Congress underway in New York City, cracked a joke about his recent bad luck with shorting as he disclosed his bet. Shares in K12 were last down 1% to $34.80, but they had dropped as low as $33.75 as Tilson spoke, according to FactSet data. Shares are up 71% in 2013 to date. K12 provides curriculum, software and other services to facilitate learning for students in kindergarten through 12th grade.