Avoid leaving China during the semester, your original visa (in most cases an F-visa valid for 180 days) will become invalid.
In order to return you need a new visa valid long enough so you can participate in the exams. Only in cases of emergency (passport loss, sick family member) or family events (weddings, funerals, etc) and other such important cases, the Zhexuexi will assist in getting the paperwork for a re-entry visa in Beijing. In all other cases you have to apply back home or go through a visa agency.
You will have need the following documents:
– JW202 (original and copy)
– PKU letter of invitation (original and copy)
– passport (original and copies)
– 1 passport photo
– Student ID
With these documents you have to go to the Vice-Dean of the Zhexuexi. He can then inform the PKU office in charge of short term students that your leave of absence is granted. Once this is done you can go to the PKU-Visa help desk (in Shaoyuan Bldg. 3, 1. door on the right side of the entrance) to process your visa application for you. They will charge you a fee (RMB 60) and the Visa Fee upon pickup (about 10 days later!).
PKU-Visa Help Desk, Shaoyuan No 3 (map)
As a student you have to go through the PKU Visa Help Desk since your original visa is a student visa. You can not go directly to the Beijing Public Security Bureau (PSB).