There is no evidence that attrition increases or recruiting becomes more difficult in schools that make the change.
In fact, in 2011, the National Education Association, the teachers' union, issued a resolution on Adequate Rest: "The National Education Association believes that overall health and performance are best achieved with adequate rest on a regular basis. The Association supports school schedules that follow research-based recommendations regarding the sleep patterns of age groups. The Association further supports programs within the education framework that promote understanding of the importance of adequate rest. (2011)."
Hundreds of districts in 40+ states have changed start times, every major medical organization and the teachers' own union agree that the evidence overwhelmingly supports later start, and several schools in Fairfield County have changed or are in the process of changing. It is also important to keep in mind that New Canaan has just as many elementary and middle school teachers commuting for later start and dismissal times and we don't hear about recruiting issues for those teachers. Teachers are aware of the trend toward later start times, both nationally and here in Fairfield County, and they surely recognize that, given the trend, basing an employment decision on the start time of a school would be short-sighted.