The TSI is an on-demand assessment. You may take or retake it at any time to determine college readiness. Students must take a TSI exam and have scores on file before they can enroll in college-level classes while in high school.
Demonstrate college readiness prior to enrolling in a college credit-bearing course - it can also replace your STAAR EOC test in some subjects!
http://www.collegeforalltexans.com/index.cfm?objectid=63176344-FFFQA-217B-60C9A0E86629B3CA
Costs vary according to institution.