In 2008, Jack Wertheimer presented data on “supplementary schools for Jewish education”. According to the report:
“For much of the past century, the large majority of American Jews receiving a Jewish education attended part time schools…. In the first half of the twentieth century, supplementary schooling was conducted mainly under the auspices of communal institutions. But by the middle of the century, the dispersal of Jews to suburbia and the explosive growth of synagogues brought an ever larger population of Jewish children into congregationally-based supplementary schools.”
The study found that Chabad schools make up about 13% of all Jewish supplementary schools in the United States.
Wertheimer says that Chabad numbers are actually undercounted because “we are missing more data on Chabad schools than in any other category. The central office of Chabad claims to have information on the existence of 380 so-called Hebrew schools operating under its banner in the U.S. alone”. The study reported just 222 Chabad schools.
Wertheimer, Jack. A Census of Jewish Supplementary Schools in the United States: 2006-2007. Avi Chai Foundation. August 2008: http://www.bjpa.org/Publications/details.cfm?PublicationID=3529