The Office of the Basic Education Commission (Obec) will start its plan to reduce class sizes in both primary and secondary levels next term.
Obec secretary-general Kamol Rodklai said the idea is that with fewer students to teach, teachers can coax a better performance from them.
Mr Kamol said Obec wants to scale down the maximum number of students per classroom from 50 at present to 40 for secondary schools and to 30 for kindergartens and primary schools. Schools will start cutting classes next term.
"Classrooms are packed, especially in top-ranked schools. We want to limit class numbers to improve education quality," he said.
However, he admitted that some schools, especially top-ranked ones, might not be able to reduce class sizes by 10-20 students at once due to their popularity.
They will be allowed to move to smaller class sizes over five years.
Mr Kamol said the number of students who fail to win a place in top-ranked schools is likely to increase as school rolls are also likely to fall.
Obec will ask popular schools to set up networks with nearby schools, so they can send those students to study at allied schools instead.
Meanwhile, deputy government spokesman Sansern Kaewkamnerd said that the Education Ministry plans to reduce Ordinary National Educational Test subjects from eight to five starting this academic year.
Maj Gen Sansern said subjects to be tested in the central admission system are English, science, mathematics, Thai and social studies/religion and culture.
He added that schools will have to test their students on the other three subjects themselves.