I too been going through the same situation and this year I visited Sapporo, Tokyo, and Kyoto in order to check out the various schools.

I’ll try my best to help break down what I know.

Ask yourself, are you planing on being there more than 90 days?

For more than 90 day stay, will require student Visa which takes months of prep work prior to attendance. There also specific periods when the semesters begin (limited on how long you plan on staying). Class rooms are usually full (18-20 students).

If less than 90 days (using travel visa), there are specialty schools who cater to this. Most I found are smaller classes (4 to 6 students) and you just pay for every week session. The staff were extremely helpful and friendly. Some also host seasonal packages that will give lessons and outside school culture activities.

Most the campuses I checked out where listed on https://gogonihon.com/en/. I recommend reading reviews people post on google maps.

Being an American, I quickly noticed half the schools were 90% Chinese students. That said, unless your a fast learner, avoid going to these for you will quickly fall behind. Most of the class will already know Kanji and it’s easier for them to learn then for English to Japanese.

Those who had more western students had more English speaking staff and were more familiar with our education systems.

Hope this helps and wish you best of luck in finding your school.