Can I visit Japan for a week on a single entry student visa and come back to China?

Ana, if you hold X2 type, once you exit PRC, you need to apply for a new X2 type to re-enter PRC.
But if you hold X1 type and have got the residence permit, you can use the residence permit to exit and enter PRC for many times. In this case, you can visit Japan for a week and then come back to PRC.