Mo. Rev. Stat. § 168.106: Indefinite contract, what affects.

The contract between a school district and a permanent teacher shall be known as an indefinite contract and shall continue in effect for an indefinite period, subject only to:

(1) Compulsory or optional retirement when the teacher reaches the age of retirement provided by law, or regulation established by the local board of education;

(2) Modification by a succeeding indefinite contract or contracts in the manner hereinafter provided;

(3) The death of the teacher;

(4) Resignation of the teacher with the written consent of the school board;

(5) Termination by the board of education after a hearing as hereinafter provided; and

(6) The revocation of the teacher's certificate.