5.1. For the purposes of the application of section 28.5.4 of the Act, the agreement between the teacher and his employer becomes void by reason of any of the following circumstances;(1) the time worked is less than 40% of the regular time of a full-time teacher holding such a position;
(2) the teacher voluntarily ceases to participate in this plan during the first year of his participation in the agreement;
(3) a teacher eligible for a pension does not cease participating in this plan on the expiry of the period agreed upon.