National Education Corp. was the second most active stock on the New York Stock Exchange on Tuesday, as investors raced to sell their shares in the Newport Beach company. NEC, which reported a second-quarter loss Monday after the market closed, ended Big Board trading Tuesday at $14.25 a share, down $2.25. Nearly 2.6 million shares changed hands, up from 128,400 the day before. Last year, the company's stock sold for up to $29 per share before starting a long decline. NEC, the nation's largest provider of educational training services, reported a loss of $1.5 million for the quarter ended June 30, contrasted with earnings of $7.6 million in the second quarter of 1988. Revenue dropped 8%, to $96.6 million.