U.S. charter schools are issuing a record amount of municipal debt, with Texas leading the charge as borrowers rated close to junk tap a program that gives their bonds top credit grades.

The institutions, privately run with public funding, have sold $1.6 billion of securities in 2014, data compiled by Bloomberg show. That’s more than all of last year and the most in Bloomberg data beginning in 2007. About $464 million has come from Texas, which for the first time in April backed a charter-school deal with its Permanent School Fund. The state-run pool guarantees bonds, lending the debt the AAA grade that Standard & Poor’s accords Texas.