Today's LD NewsNational Public RadioPreschool is getting a lot of attention these days. President Obama and mayors across the country are touting preschool as an important investment in the economy. As policymakers weigh the costs and benefits of "preschool for all," they're trying to figure out what actually works in the classroom. One of the places they're looking is Boston. School officials say their system of intensive teacher coaching is a key to classroom success. Boston uses coaches as part of its larger push for preschool quality. The district has installed a rigorous curriculum and requires teachers to get masters degrees. Academics have known for some time that quality matters, but much of the research has focused on small-scale programs. The Boston program is different. Last year, a Harvard University study found big gains in math and vocabulary in Boston, where the preschool system spans 68 schools.