As of Monday, December 28th, the Japanese government put into effect tighter border restrictions for most foreign nationals in order to prevent the spread of the new coronavirus. This restriction is put in place at least until the end of January 2021.
For those who are student visa holders already and were planning on coming to Japan for January 2021 enrollment, it is possible to arrive in Japan if your flight lands before January 4th at 00:00, assuming that you have all of your other documentation in order. In this case, it is likely that you will need to arrange your own accommodations to carry out your 14-day quarantine as well as private transportation to the accommodations from the airport.
NOTE: Neither buses nor taxis are considered private transportation.
This new restriction builds from the previous border control measures, which previous has banned all short-term trips to Japan, such as tourism. Added to this list now include all long-term visas, such as student and working-holiday visas, for the exception of residents of Japan and of course Japanese citizens.