Schools with clear purpose

This article reflects research that claims that students do well where schools have a clear mission and purpose. Religious schools are examples of this but there are others, eg schools with foci like the arts, languages, gender, etc.

http://www.economist.com/research/articlesBySubject/displaystory.cfm?subjectid=526356&story_id=16945031

I am particularly interested in the implications for public education. If parents choose, does this weaken a generalist approach to education that promises excellence in education for all students. Do such schools, though publicly supported, create enclaves of selected students? Do parents with higher education levels garner even more advantage for their children?

I'd like to hear your views.