Each teacher employed under a continuing contract is entitled to continue in the teacher's position or a position with the school district for future school years without the necessity for annual nomination or reappointment until the person:
(1) resigns;
(2) retires under the Teacher Retirement System of Texas;
(3) is released from employment by the school district at the end of a school year because of necessary reduction of personnel as provided by Section 21.157;
(4) is discharged for good cause as defined by Section 21.156 and in accordance with the procedures provided by this chapter;
(5) is discharged for a reason stated in the teacher's contract that existed on or before September 1, 1995, and in accordance with the procedures prescribed by this chapter; or
(6) is returned to probationary status, as authorized by Section 21.106.