I have a Chinese Visa but I am only allowed to leave the country once

You have this completely backwards. The 144-hour transit is for those entering China and staying for up to 144 hours, not so you can leave the country for less than 144 hours and come back as that will be outside immigration's legal jurisdiction. In your case (and any case) once you have left Mainland China for any reason or length you need a new X2 visa to re-enter, plain and simple. If you have an X1 then you may want to get your resident permit first before traveling.

Oh I see. Thanks for clarifying!

I think I meant to ask this question. I want to go on a cruise to Japan on December 21. Exiting China would mean by student visa would become null. But I plan to go to Japan until Dec 26. Could I then stay in China for 2 days, AND THEN return to the United States on the Dec 28? Does this 144hr exemption apply here?

Thanks very much for the reply, sorry I am ignorant on this topic.

In that case it applies, providing that you do not leave Shanghai, Jiangsu or Zhejiang in these two days and your return flight does not stop anywhere else in Mainland China.