MY SO is a professional tutor, and she often has clients for standardized testing prep.

From what I have gathered it seems a lot of students really need a confidence boost. Once they realize they are capable to doing well, they tend to improve their performance.

There is also like tricks and gimmicks, but that is a different discussion.

You can get the same improvement by reading a test prep book. You need to know the 3-4-5 triangle, this list of word definitions, a handful of other stuff. How much time you have. Now take a few practice tests, also freely or cheaply available anywhere. All of it should be a review of methods and skills you already have from school. That is the point of the test after all.

To me, those test prep places always seemed to be struggling with legitimacy. It's not like hiring a tutor to help you learn difficult concepts for the first time.

Our [public] school gave a 1-hour class on the basics of the SAT test mechanisms and how to get your score to what I'd call "reflective of your ability". If kids are showing up to the SAT barely knowing how a #2 pencil works, that's bad. If they've had 1 hour on "if you can eliminate even a single answer that you know is not correct, do not leave the question blank" and "D means that no can determine which is greater not that you have no idea which is greater", that puts them in a position to demonstrate their ability and that's well within the reach of any school district that gives half an ounce of care for their students.

I attended prep courses and they didn't have an impact on my scores, measured by before and after practice testing (~1500 in the late 90s).

What did help is that I was encouraged to read a lot from a young age, which seems fairly accessible to most families.

Support your local libraries.

If SAT prep was effective, then it would be an educational miracle, because a good SAT prep course is under $1,000. American schools spend 10 times that much money per student every year and barely move the needle.

Test prep is effective at getting you an accurate score.

One bit of knowledge improved my ACT score from 27 to a 33: internalizing how long I have for the tests. I was so used to being able to finish school tests with a comfortable amount of time remaining, so I took the test at a casual pace which means I didn't finish. The second time, I sprinted through the test as fast as possible and my score improved dramatically.

I doubt I could have done better with more coaching or prep.

Similarly, a single 30-minute lesson taught me what I was doing wrong on the ACT Science section (trying to understand the science instead of treating it as 'find the answer in the text'). Brought my overall score from 32 to 35 (I was already a good test taker, but those 3 points made a big difference),

I legit think that my parents' successfully instilling a strong sense of fair play and honesty in my from a young age hindered me more than a little. The world's simply not like that. The ones who win take every advantage—if a test is trying to measure you, you'd damn well better study its "game", because you're competing against kids who did and they will receive zero negative consequences for doing so, only benefits, even if doing so seems against the spirit of the thing and feels dirty.

... which is part of why having parents who have themselves succeeded at these things is an advantage. Mine were from poor (quite poor, in one case) backgrounds but were worked-hard-and-eventually-did-OK sorts, not early-success sorts. So they taught me fairness and honesty in all things, and didn't know they should clue me in that some things are just bullshit games that you should do whatever you can to beat short of taking too-large risks. I mean, they hadn't even played most of those games, so how could they know?

I bet it would never occur to kids from an upper-middle-class background (doctor parents, lawyer parents, business exec parents, that kind of thing) that test prep is a bit dirty. Wouldn't even cross their minds. It's just what you do, obviously.

Those techniques are things like skipping questions or calculated guessing which can help but certainly won't get you into elite score territory.

They increase speed, which buys you time to work on harder problems. I bet a nonzero number of 2nd tier students could get a 1st tier score if given an extra 10 mins on each section.