A teacher is indicted for Improper Relationship Between Educator and Student. She was sending sexually explicit text messages to her high school student. She plans to enter a plea of guilty and be placed on probation. Does she register as a sex offender? I don't see 21.12 listed in TCCP 62.001 (5). Seems like she should be required to register. Has anyone addressed this issue before?

You read it right. The improper relationship between educator and student offense under 21.12 is not a sex offender registration offense.

Since you didn't indict the teacher for online solicitation of a minor under 33.021, I would assume that the student was not under 17? If the student was, that would be registration offense and one that it sounds like the conduct you described would fit.