Thing One: The exams are generally intended to measure how good you are at performing certain tasks (working out or knowing the correct answers to questions). On a given day, etc.
Thing Two: the exams are actually used to estimate how good you might be at other tasks (studying for a degree, doing a job, being "smart"). Of course SATs and the like are not great at estimating those things. For that matter intelligence doesn't easily admit being measured at all. But it's one piece of evidence and people seem to care about them.
So, get one consideration out of the way. Is it unethical to train for the test at all? Maybe, because that makes it a less good proxy for general ability. But you can train for the test with or without a tutor (perhaps more effectively with). Just practicing questions makes a significant difference. A good (but not off the charts) SAT score from someone who has never seen a SAT-style test before is impressive. A good (but not off the charts) SAT score from someone who has seen them before just means they're capable of studying. I think in this question we're not interested in whether receiving tuition is unethical. We'll assume it isn't and then we're interested in whether paying for it is unethical, right?
Cheating on the test subverts both Things. The first because what ends up getting measured is not anything you can work out for yourself (aside from avoiding getting caught), but something someone else can do for you and pass you the answers to fill in. The second because you won't always be able to cheat at your job and you won't be able to cheat at being "smart".
Paying for a tutor doesn't subvert the first thing at all. You really can work out the questions on the day. The reason you have that ability is in part because you have money to spend on the problem of learning it, but the test doesn't set out to measure how the ability was acquired, only that you have it. So from the point of view of standard testing, cheating is unethical and tutoring is not.
Paid tutoring may well subvert the second Thing, since you won't always be able to hire a tutor to teach you how to do any task you need to complete in life. SATs are quite predictable (intentionally so), challenging tasks in real life aren't. You can reasonably argue that if someone uses SATs (or GMAT, or IQ, whatever) as a proxy for some other activity then it's their problem to figure out what correlation there is (if any) between what ability you need to pass the test and what ability they really want.
Personally I hold to some socialist principles, so I think it's regrettable that opportunity and power in the country I live in is distributed primarily according to wealth. Which in turn is primarily inherited, at least when talking about people's early education, where any benefits like this coaching are typically paid for by parents. It's possible that this question is inspired by similar principles, in which case I sympathise, but the only way to apply those principles is to consider what it is and is not ethical to acquire through wealth. Different people will have different answers. Education? The ability to pass silly standard tests? A house to live in while you're educated? You can acquire free time through wealth, does that make it unethical after all to practice for SATs? If you think it's unethical to acquire advantage through wealth, sure it does.
In a perfect free market, people for whom a SATs tutor is a worthwhile investment would be able to borrow money to do so regardless of their economic background, and get the same benefit. In a perfect socialist state, access to education and jobs would not be based on wealth at all (indeed, 'wealth' could be meaningless or at least would not significantly vary across the population). Each system has its own idea of what equal opportunity means.
The actual world is neither of those things: some people can get an educational advantage by paying for a better education and others cannot, purely by the wealth of their parents. Society is not fair. Tutors are part of that privilege. As such I think they are part of the broader question of whether it is ethical to be rich, but there's nothing special about tuition in particular that makes it less ethical than spending money on anything that gives you an advantage of those who can't afford it.
It's probably less reputable to pay for coaching to a syllabus like the SATs that (for the purpose of this question) we assume to contain no real knowledge, than it is to pay for tutors to teach you something more educationally worthwhile. The more people train directly to a test, the less valuable the test becomes under Thing Two. But I don't think either is held to be actually unethical except by those who want the student's wealth taken out of education more generally.