90days visa for use with language school

I would like to stay in Japan for 6 months. Not to work, but to study at a japanese language school and get better at it. I am an American citizen which gives me visa free 90 days stay. Since I'm not going there to work, Will it be a problem to go out of Japan and then come back again for another 90 days to complete my plans of 6 months stay ?
Or can I just apply for an extention with the immigration and show them that I've enrolled in a language school? Or should I apply for some type of visa related to language school before I leave USA ?

Curious, most legit language schools should be able to sponsor you for a "pre-college student visa or college student visa" depending on the school.

Of course you need to show that you are financially able to support yourself to immigration.

No you can't apply for a extention as a us citizen under visa waiver.
Going and coming back in 90days is always risky.

Not to mention you will lack many things you can get with a student visa.

For example with a student visa you can open a japanese bank account, get a japanese cell phone, health insurance etc.

All of this is not possible (most of the time) with just simple tourist visas/temporary visitors.

90 days visa free is only for touring the country then? If I go in the second time and tell immigration that I'm going there to a language school I will be banned ? I want to go there to learn the language and not to work so technically it should be OK right?

You can enter Japan under a 90 day visa/visa waiver or as a tourist to go to language school. What i'm saying is that if you return for another 90 day for whatever reason it is, even if it is to sight see more, you might be asked questions. It is really up to the immigration officer at the time to make a decision to allow you in on another 90 days or not. Some will after reviewing it, some won't, its really discretionary based on the information they have at hand and their judgement.

Again if you want to stay for six months (or more) its a lot easier to get the language school to sponsor a student visa for you so its more hassle free for entry, plus you get the benefits listed in my other post.