"More study time means better grades."
No, no matter how much the Japanese might believe it, not after a certain point. If you mentally exhaust yourself while learning, you will more likely fail—source: Personal experience.
"A high schooler with no experience shouldn't have a real answer to that."
You can do that as a teenager, I did this as well and I went to the university I wanted and worked, at least for some time, for the company I aimed for. Then I changed to a different one which was better in almost all regards and never regretted it, also because hiking and mountaineering became very possible. I have never regretted my decisions about education and jobs.