Students that have obtained work permission are allowed to work part time in Japan. Then you can work 28 hours per week on school weeks and 40 hours per week during semester breaks. Students are not allowed to take any jobs related to night clubs or adult entertainment. This even includes being a DJ or working as a dishwasher in a night club.
Most students work in shops or restaurants where you can earn around 1000 Yen per hour. However it can be hard to find a job before you speak decent Japanese. The best paid jobs for students are often to work as a teacher in your native language. Either at a school or just give private conversation lessons in a café then it is not unusual to earn up to 3000 Yen per hour.