Did you ever take the PISA exam when you were in school?
Neither did I. My children never took this exam in any of their years in schools. I also never had a student take this exam in 21 years in the classroom. So how valid is the exam? It is like a poll of 300 people being used to predict a presidential election and the only people polled live in the same block in the Bronx!
Do you think that might be best change in the current method in schools and making the tests the judge to give sentence of who passes and who doesn't pass the grades?
This is to say, the student fails three subjects and he must repeat the grade again.
Like in other countries, the student who won't pass the test then won't pass the grade. He can repeat the same grade up to certain age. To avoid having a 18 years old student in a10th grade class, the student will finish his learning at night school classes. These night school classes give him the chance to work at day time while he finish his studies.
No student get "traumas" because he didn't pass the grade, I don't know who invented such imaginary weird human behavior because he failed to pass a grade in school.
On the contrary, if he doesn't make the effort and fails passing the grade, then he will learn that he himself will stay behind the rest. Certainly tests are the judge for you to become a teacher or a doctor or an electrician. School must be the best place to start learning the way of life.
About the complications with students coming from other countries, I know there are programs in schools to make foreign students to catch up with English.
I don't think in private schools students will pass grades without passing tests, because my children had to pass those tests and make high scores in order to receive diplomas. They had lots of homework, too many in my opinion. The prestige of the school is in play and students in many private schools won't pass grades without knowing the subjects.
A change in the public school system must be to stop passing grades to students who are not learning the subjects. At the end, this model from public schools of allowing the ignorant passing grades is what causes the abyss between private and public schools. The consequences are seen when they become adults and the careers they can get.
Of course, there are some public schools with excellent educational records, not because these have a different curriculum than the rest, but because somehow they receive more funds (from private donators) the parents demand better work from teachers, and children come from well established families, this is to say, public schools in the rich area of the city.
Language is a barrier as you say, but I know a family where the mother is divorced, she lives with a boyfriend who has remodeling business, the two children went to a public school which is from standard to mediocre at general status. However, with their English learnt since 12 years of age when they came to the US, the boy is now an attorney and the girl now works as a social worker with Master and making close to $100,000 salary.
This is because they wanted to succeed and made the effort.
Then, there are many factors to be considered in the school system, some of them which are beyond passing the school tests. To start, teachers must not limit themselves to the giving of lessons and performing tests but part of their job is to encourage students to become successful in life. No need to investigate the private life of each pupil, but always giving them tips, paths, ideas of how to become better. This requires no more than a minute in each class.
The teacher is the fundamental piece in this chess game, his students must not be "my students" but "my babies", and taking the best care for them is a must.
Teachers must love their jobs as well, because in their hands is the future of the new generations.
Then, revolving the tests, there are the student, the teacher, the family, the social environment,. One can't ignore anyone of them when the future of the student is the central goal.
From 100 students, only a few ones will reach their goals, the rest will find the way to keep their lives in good standard, and many will just will got lost.
However, the opportunity must be always there, and this is the point: Not because the student already shows no signs of progress, then such is a reason to make him pass the grades to evade him as a burden in the system. Definitively, that is a no no.
