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.