SAT prep per se is an unbelievably shitty thing for people to waste their time on. It's largely mindless and uninteresting, stifles rather than encourages curiosity, emphasizes judging people by substantially arbitrary numerical scores, gives the false impression that some people are inherently better than others, and, in the medium to long term, is a grossly inefficient way to improve performance on the SAT.

If you want your own kids to get a high SAT language score when they are high school students, the top things you can do to help them are: (1) read aloud to them when they are very young, as much as you have time for, ideally choosing excellent books of wide variety, (2) keep reading aloud to them when they are older, (3) encourage them to read for pleasure, (4) converse about the world with them, without condescending.

If you want your own kids to get a high math score, (1) surround them with technical materials (construction toys, logic puzzles, board games, circuit parts, programmable robots, or whatever) and play with them together – or if on a tight budget, improvise materials from whatever you have at hand, and (2) spend time working non-trivial word problems one-on-one. Start from https://archive.org/details/creativeproblems0000lenc

If you have the personal time to do these steps, you won't have to give a shit about what their SAT score is, because it will be good enough for whatever they need it for. (Sadly as a society we don't have the resources or motivation to get every child enough listening-to-books-read-aloud time or enough playing-with-technical-materials-with-adult-help time, so we try to replace it with cheaper and more scalable vacuous alternatives like multiplication drills, spelling quizzes, and SAT prep.)

In Poland, many schools pride themselves for how good they are at preparing for end of high school exams. Nobody fails, nobody gets bad score.

Because if they as much as suspect you will fail, they will not let you graduate.

But statistics are kept clean :)

you might find this interesting : https://www.morethanascore.org.uk Says a lot of what you're saying, with some statistical evidence to back it up