An anonymous group of donors has given more than $12 million to a small Jewish high school in Waltham, MA., providing a boost not only to the school, but also to the world of Jewish philanthropy, which has been jolted by the twin shocks of the recession and the Madoff scandal. The contribution to the 315-student Gann Academy appears to be one of the largest ever to a local Jewish day school, and the first major gift in the local Jewish world since the recession hit.