When planning to travel outside of the United States and wish to re-enter the USA as an F-1 student, you will need to obtain a DSO's signature on the 2nd page of your I-20. 

In order to obtain this signature, you will need to show proof of enrollment for the following fall semester.  Visit the ISP office to fill out the Travel signature request form and bring your current I-20 with you.  Plan ahead and drop off your request at least 10 days prior to leaving the U.S.    

You will need to present the following documents at the U.S. Port of Entry:  a valid passport, visa and I-20 with travel signature. It is also strongly recommended you bring other supporting documents such as proof of enrollment for the following semester.