One way ticket to Japan (complicated)?

I will be moving to Japan soon as a language student for less than the 90 days (US tourist visa). My issue is this: I would like to purchase one way ticket to Japan, then depart from Japan (after my season class session) to Guam for the holidays...then return back to Japan for a second class session (again, within the 90 days). It sounds and seems complicated. Can it be done without problems relating to customs or immigration or re-entry into Japan? Will it be difficult to purchase one way ticket initially? Is this not as difficult as it seems?

I recently moved here, Japan, from the US and on a one way ticket. The US check-in agent checked my passport for my Visa. So you might want to book your Guam flight at the same time you book your Japan flight. About returning to Japan after Guam, I don't think you would have any problem there. But since I have never done it don't quote me on it.

Most airlines won't allow you to board the plane (in the US) with a one way ticket unless you have a visa that isn't temporary/tourist.

You have to have some means of leaving the country.

Check for a round trip ticket that's good for greater than 6 months. Some are good for up to a year. An agent could help you with this. The extra cost is still probably less than two 1-way tickets.

I say 'probably' because I flew LAX-NRT once on a one way that was actually LA to Seoul. I just got off in Narita and didn't get back on. It was cheaper than a RT at the time and I was given a tourist visa at immigration.

Check on the amount of time you must be out of Japan to return on a tourist visa. 2 weeks comes to mind but that's just what I remember w/o looking anything up.