TRS is years of service + age = retirement "rule."

If you leave the profession 5, 10, 15 years early, you just have to wait longer to claim full benefits. If you leave so early that you can't reasonably claim full benefits (say, leave teaching in your 30s after 10 years, so you'd need to be in your 70s to meet Rule of 85) then just pull the money out of TRS and re-invest it in a 401k.