Tex. Educ. Code § 37.0021. Use of Confinement, Restraint, Seclusion, and Time-Out.
...

(f)  For purposes of this subsection, "weapon" includes any weapon described under Section 37.007(a)(1).  This section does not prevent a student's locked, unattended confinement in an emergency situation while awaiting the arrival of law enforcement personnel if:

(1)  the student possesses a weapon; and
(2)  the confinement is necessary to prevent the student from causing bodily harm to the student or another person.