Shares of software-as-a-service education and assessment platform company Instructure (NYSE: INST) took a big hit Tuesday, falling as much as 21.8% and down 16.3% at the time of this writing.The stock's decline comes after the company's second-quarter earnings release on Monday. Though Instructure's revenue of $50.1 million and non-GAAP loss per share of $0.24 were better than expected, its lower-than-expected guidance for the third quarter could have worried some investors.