Luria Academy in Prospect Heights, Brooklyn, which started the new school year this week with 140 students (up from 107 last year) and a newly expanded facility, is one of more than 40 Jewish Montessori schools in North America. More than 20 have, like Luria, opened within the past decade, forming a small but growing movement, a bright spot within a Jewish day school world where (with the exception of the ultra-Orthodox community) flat or declining enrollment is the norm.