SOUTH BEND, Ind. (AP) — Weeks after Indiana began the nation's broadest school voucher program, thousands of students have transferred from public to private schools, causing a spike in enrollment at some Catholic institutions that were only recently on the brink of closing for lack of pupils.It's a scenario public school advocates have long feared: Students fleeing local districts in large numbers, taking with them vital tax dollars. In at least one district, public school principals have pleaded with parents not to move their children.Under a law signed in May by Gov. Mitch Daniels, more than 3,200 Indiana students are receiving vouchers to attend private schools. That number is expected to climb significantly in the next two years as awareness of the program increases and limits on the number of applicants are lifted.