North Carolina leads the nation in the number of teachers who have earned certification from the National Board for Professional Teaching Standards.

Nearly 21,500 teachers in the state have the national certification, with 616 additional teachers gaining the endorsement last month.

North Carolina accounts for nearly one-fifth (18 percent) of all teachers nationally who are certified by the teaching standards organization.

The certification is based on a rigorous performance assessment that typically takes from one to three years to complete and measures what accomplished teachers and counselors should know and be able to do.

Teachers in North Carolina who achieve certification receive a 12 percent salary supplement to their regular pay.

Certification by the National Board is the highest credential in the teaching profession, and participation is voluntary.

The National Board for Professional Teaching Standards also ranks the top 50 public and private universities and colleges with the highest number of alumni with the national credential. Appalachian State University was first with 2,049.