Churches Using Public School Buildings

A recent article in the Boston Review discusses "church planting" by evangelicals in Boston.  Church planting involves creating small, low-cost congregations, often using underused buildings.  A new approach is using public school buildings for church meetings, which can be rented at little-to-no cost.  Read the article.  Why is church planing an effective strategy?  What challenges, legal or otherwise, do these churches face?