If you’re a teacher, and engage in sexual relations with someone who is a student at the school where you work, you can be charged with improper relationship between educator and student.  Even if the student is over 18 years of age, and the sexual contact is consensual, you can still be arrested for improper relationship between educator and student.  Also,

the fact that the student is not enrolled in any of your classess is of no consequence, under Texas law.  You can still be charged with this offense.

However, it is a defense to prosecution if the teacher and student were husband and wife at the time the sexual conduct occurred.  Likewise, you cannot be prosecuted for improper relationship between educator and student if the relationship was in existence prior to the time you became a teacher at the school, and the age difference between the two of you is no greater than three years.
Texas Penal Code 21.12.  IMPROPER RELATIONSHIP BETWEEN EDUCATOR AND STUDENT.(a)  An employee of a public or private primary or secondary school commits an offense if the employee:

(1)  engages in sexual contact, sexual intercourse, or deviate sexual intercourse with a person who is enrolled in a public or private primary or secondary school at which the employee works;

(2)  holds a certificate or permit issued as provided by Subchapter B, Chapter 21, Education Code, or is a person who is required to be licensed by a state agency as provided by Section 21.003(b), Education Code, and engages in sexual contact, sexual intercourse, or deviate sexual intercourse with a person the employee knows is:

(A)  enrolled in a public primary or secondary school in the same school district as the school at which the employee works; or

(B)  a student participant in an educational activity that is sponsored by a school district or a public or private primary or secondary school, if:

(i)  students enrolled in a public or private primary or secondary school are the primary participants in the activity; and

(ii)  the employee provides education services to those participants; or

(3)  engages in conduct described by Section 33.021, with a person described by Subdivision (1), or a person the employee knows is a person described by Subdivision (2)(A) or (B), regardless of the age of that person.

(b)  An offense under this section is a felony of the second degree.

(b-1)  It is an affirmative defense to prosecution under this section that:

(1)  the actor was the spouse of the enrolled person at the time of the offense; or

(2)  the actor was not more than three years older than the enrolled person and, at the time of the offense, the actor and the enrolled person were in a relationship that began before the actor’s employment at a public or private primary or secondary school.

(c)  If conduct constituting an offense under this section also constitutes an offense under another section of this code, the actor may be prosecuted under either section or both sections.

(d)  The name of a person who is enrolled in a public or private primary or secondary school and involved in an improper relationship with an educator as provided by Subsection (a) may not be released to the public and is not public information under Chapter 552, Government Code.

Added by Acts 2003, 78th Leg., ch. 224, Sec. 1, eff. Sept. 1, 2003.

Amended by:

Acts 2007, 80th Leg., R.S., Ch. 610, Sec. 1, eff. September 1, 2007.

Acts 2007, 80th Leg., R.S., Ch. 772, Sec. 1, eff. September 1, 2007.

Acts 2009, 81st Leg., R.S., Ch. 260, Sec. 2, eff. September 1, 2009.

Acts 2011, 82nd Leg., R.S., Ch. 761, Sec. 3, eff. September 1, 2011.