My new book, From Preschool to Prosperity, includes a discussion of the controversy over whether pre-K programs get better results if they use credentialed teachers. The controversy arises because the research on this topic is mixed.
However, even though the research is mixed, the pre-K programs with the best evidence of success tend to use credentialed teachers paid public school wages. This is true for Perry Preschool, the Chicago Child-Parent Center program, and the universal pre-K programs in Boston and Tulsa.
Obviously more research on the impact of different teacher credential requirements is warranted. But if one wants greater assurance that a pre-K program will be effective, it seems prudent to follow the example of programs with demonstrated success, and seek to hire credentialed teachers and pay wages close to the public school standard.
It seems likely that paying higher wages will reduce teacher turnover. Most studies of teaching suggest that new teachers are less effective than teachers with some experience, so higher teacher turnover would be expected to lower program quality.
It is possible that the mixed research on teacher credentials is related to the interaction of credential requirements with salary standards. Imposing credential requirements, but then paying salaries below public school levels, may increase teacher turnover by restricting the pool of eligible teacher candidates to those who have the alternative of teaching in the K-12 system, but then underpaying them relative to their alternative opportunities.
In addition, the impact of teacher credentials and salaries may depend on the size of the pre-K program. A small program may be able to find some dedicated staff who will work at the program despite the low pay. At a large scale, this strategy is harder to carry out.
Given the importance of quality in pre-K, a wise strategy is to err on the side of overinvesting in quality, until further research clarifies what program features are essential for quality.