You have greencard you have to study outside of the USA and you are going to stay out of the USA for more than six months?

Maintaining Permanent Residence

You may be found to have abandoned your permanent resident status if you:

-Move to Another Country intending to live there permanently.

-Remain outside of the US for more than one year without obtaining a reentry permit or returning resident visa. However in determining whether your status has been abandoned any length of absence from the US may be considered, even if it is less than one year.

-Remain outside of the US for more than two years after issuance of a reentry permit without obtaining a returning resident visa. However in determining whether your status has been abandoned any length of absence from the US may be considered, even if it is less than one year.

-Fail to file income tax returns while living outside of the US for any period.

-Declare yourself a "nonimmigrant" on your tax returns.

Source: www.uscis.gov