North Carolina’s public school classrooms continue to lead the nation with the most teachers certified by the National Board of Professional Teaching Standards, according to results released by the organization Tuesday.

The state now has 19,799 National Board-certified teachers, accounting for almost one-fifth of the country’s total. More than 20 percent of the state’s teachers have this certification, which is considered the highest credential in the teaching profession.

Wake County is the top district in the nation, with 2,299 National Board-certified teachers. The Charlotte-Mecklenburg Schools system is third with 1,897.

Across the nation, there are 102,237 National Board-certified teachers.

The process is rigorous and voluntary. North Carolina teachers who achieve certification receive a salary supplement of 12 percent.