One of the state's most-influential business organizations is backing a flexible high school graduation plan it says will better prepare students for college and the workforce of tomorrow.   The Texas Association of Business has been a strong supporter of raising academic standards in public schools.   It has been a backer of STAAR, a state standardized test implemented last year. STAAR is required for graduation and designed to be far more-difficult than previous exams of its kind.  But the association's president, Bill Hammond, is holding a Wednesday news conference in Austin to present a "new, flexible graduation plans for students."  Hammond promises the initiative will promote post-secondary education and workforce readiness while also "maintaining a strong accountability system that focuses on improving student performance and education quality."