The low birthrate is now starting to shrink high-school classes across the nation and in some cases shutting down some subjects completely.

Education Ministry data last week show that the number of new students entering high school this year dwindled by around 60,000 or 11 percent. A total of 525,300 students finished middle school last year and are entering senior high school this year, compared to 591,845 last year. And next year, only 460,000 are expected, down another 12.2 percent.

"We've been reducing classes since 2004 and will have to sharply lower them again this year," a ministry spokesman said.

The drop has prompted major changes. The same thing happened in middle schools in 2014 and elementary schools in 2008.

The student-teacher ratio fell to 27.4 last year. The number of teachers allocated to middle schools has dropped since 2014, when it peaked at 113,349, reaching 109,525 last year. Now it is the turn of high school teachers to be weeded out.

