I did a 1-month program in Kyoto 4 years ago. It was fun, but they only had beginner and advanced. Me and my friend was somewhere in the middle so we joined the advanced class first. Couldn’t understand a thing of what was going on, so we were put in the beginner class which was way too basic (how to write Hiragana etc…). Beginner class only had 1 guy, so we raised the level of the entire class and others joined in from the advance class. That poor 1 guy…
Anyway, there are many short term language schools around. Google around or get help from the embassy. I think our course was about 60 000 JPY / month + accommodation etc. (own arrangement).
It is very good for an entrance into Japan as you can make some friends and contacts, however not all schools are very professional. My teachers were just there a couple months for training through their university program, so they weren’t really experienced. However, it was ok as we did a lot of activities such as calligraphy, daytrip to Hiroshima, making food, doing a play, went to matsuri together etc.
I really do recommend to go with someone else if you can. Japan is lovely, but I was happy to have my friend there to share the “lost in translation” experience.
If you have any questions, let me know