How can I apply for Citizenship if I had been out of the country for more than a year because I had to go to school abroad?

You cannot apply for citizenship if you have been gone for a year.  Any absence of more than 180 days total (or 90 days consecutive) would require you to wait another 3 years or 5 years depending on how you gained your permanent resident status.  What's more, you may have abandoned your permanent resident status after such a long absence.  You should talk to an attorney about your status.