Either the O-net tests are testing the wrong thing or the teaching is very poor. There are now proposals for those very teachers to alter the tests.
I come from England, and the tests I passed were set by the (now defunct) Northern Universities Joint Matriculation Board. In other parts of the country, the tests were set by other university boards. And the tests were marked by external examiners. Those days are long gone, and the standard of education there has fallen dreadfully, though the marks get better and better each year!
The universities set the exams because they knew what standard was necessary for the students in order that they could benefit from a university education. May I suggest that Thailand adopt a similar system.
Roger C Haslock
Bangkok