Where do you start buying your own home?

[zeroshiki]

You can look up the schools' standardized test scores I believe (I don't have children so I don't know how valid this strategy is).

"Name of school" 偏差値 should get you some hits.

(I actually googled and found this site: https://www.minkou.jp/primary/ which is a site with school reviews I guess)

[RetireJapan]

How do you know what area has "good schools". Are there rankings for elementary, junior, and senior high schools?

Not sure. You mainly need to know the catchment areas for the schools (ie if there is a lot of public housing you will get more poor children from less 'successful' families). In my experience it also varies wildly each year as the students and teachers change. The individuals in the classes and in the staffroom make a huge difference.

[Ori]

Very foolish question here. How do you know what area has "good schools". Are there rankings for elementary, junior, and senior high schools? Is there any way to view this info on a map or overlay?

My wife I and I are pretty settled on public elementary. However, we are prepared to pay for Junior or senior high school.

This is actually a good question which I don't have a good answer for.
I'm planning for public elementary and Junior, and senior high doesn't have to be close to home, so basically I'm only looking at quality of public Junior high.
The way I've been approaching this so far was exactly how other ppl have already answered - check reviews at www.minkou.jp, lookup 偏差値 rankings (not always easy to find), check if there is a lot of public housing nearby (市営住宅), avg income in the area. Reviews indeed have less value for big schools.
As a rule of thumb, the more affluent the neighborhood is and the more expensive the land is, the better the school is. No surprises there.

[7-seasons.com]

I was just looking at this myself. There look to be some ratings on most schools in the popular Kanagawa neighborhoods that I looked at using google maps. Not sure how reliable it is but certainly better than nothing! Also found some areas with very few schools and/or mostly schools with bad ratings so I'm glad that I checked.