The Board of Education for Township High School Glenbard District 87 (the school board) (defendant) approved 23 teachers (plaintiffs) for a retirement plan (the plan). The school board subsequently sought to decrease the health-insurance benefits provided to the plaintiffs under the plan. The teachers sued the school board. The school board argued that the teachers should not have been eligible for the plan in the first place, because they did not also participate in a separate retirement plan, the Teachers’ Retirement System Early Retirement Option (ERO). The school board claimed that it initially had made a mistake as to the condition precedent of ERO participation and now simply sought to fix that mistake by arguing that the plaintiffs should be disqualified from the plan. The trial court ruled that the school board waived its right to enforce the plan’s condition precedent that teachers also participate in the ERO. The school board appealed.