When I used the term assessments I did so in the broad sense to include testing, exams, homework and other forms of continual assessment.
We cannot compare Thai schools with most if not all western schools. Yes I agree that inclusion is starting to work there now, but the teething problems were immense and possibly caused irreparable harm to the outside the box students during the initial phases. Since I have been teaching in Thailand I have taught in two Matayom schools (state) and one private Prathom, in all three there were between 9 and 12 classes per age group. However in only one school was there any form of academic streaming, in the others it was purely financial, i.e. those who paid the most tea money got their kids into the /1 and /2 classes those with the least or none ended up in /12.
Yes class sizes and assistance or lack of it do have an impact but it should be remembered that the old Jesuit missionary schools regularly had classes in excess of 300 and still managed to attain good results. This was achieved by instilling a sense of competitiveness in the students and having rigorous testing and thus positioning within the class. In Thailand where class sizes are around 50 there should not be a problem if there was adequate discipline and motivation and some sort of achievable goal to show to the kids. That and failure being an option, with failure ensuring a revisit to the same academic year again, with the loss of face that this would entail.
The Thai national curriculum is vague and there appears to be no oversight to ensure that it is implemented, in many schools I doubt that the envelope has even been opened let alone the document has been read. I have rarely been given an adequate syllabus and when I have it bears no resemblance to the national curriculum. With so much mal-administration its little wonder that school admins adopt the no fail policy.
The reasons for failure are numerous, inadequate teaching, facilities, motivation, lack of discipline and many more, but none of them are aided by the no fail policy except by hiding the reasons.