Whether it's a question about economics, education, psychology, sociology, or any other subject that could be relevant to politics, I think the answer would be the same: if there's an explicit connection to a government process or a public policy then it's probably on-topic. Otherwise, the question is either off-topic or there's a more suitable SE site to try migrating it to.
Just to reiterate the first line of the 'What topics can I ask about here?' page of the help center:
Politics Stack Exchange is for objective questions about governments, policies and political processes.
The 'governments' part is pretty clear, but 'policies' and 'political processes' and their own are unfortunately fuzzy. However, the rest of that page seems to suggest that the focus of the site is on the politics of governments and the politics that influence governments:
Central to the idea of this site are the nuts and bolts of policies introduced by governments...
Processes are central to how legislation is made. Questions seeking to understand the rules and processes by which policy is made in various legislatures or ruling bodies ... are wholly on topic
Just to clarify, Macroeconomics is specifically on-topic. Most public policy questions involve economic matters
In just about any policy of substance, there are particular personalities that are central to its understanding, as well as demographic data about supporters and opponents of legislation...
...answerable using polls, punditry, and other verifiable and reproducible sources.
So, to use the Why people with rich parents have higher SAT scores? question as an example, there is certainly a political angle to the subject: SAT scores affect education and wealth affects SAT scores, and wealth and education both affect voting patterns, so questions about the wealth gap in SAT scores definitely could be on topic here.
However, just asking about why the wealth gap exists isn't political enough on its own, there needs to be a more explicit connection to politics. To show how such a connection could be made, here are some examples that take the core subject of the original question (higher SAT scores for richer students) and ask about it in a way that's more firmly on-topic here:
- Politician X claimed there is a wealth gap in SAT scores because of [some reason], how true is that?
- If politicians are talking about a topic it's almost certainly political, so most questions looking to clarify, expand on, or verify what a politician said should be on-topic
- State/City/public university XYZ offers free SAT prep courses, has this affected the wealth gap in SAT scores?
- If a government entity has some policy to offer something to the public, then questions about the implementation or affects of that policy are likely on-topic
- Is there any legislation or policy aimed at improving SAT scores amongst poorer students?
- Pretty much any question about legislation or government policy should be on-topic, as well as any question with an explicit connection to that legislation/policy
In short, a question about a non-political subject, such as SAT scores, can certainly be on-topic here, but only if the focus of the question is how that non-political subject affects something political. Each of the these example questions take the non-political subject of the original question and explicitly frames it in a political context, and I believe you could do this to pretty much any non-political subject in order to make it on-topic here.