Federal Withholding with TRS

I am a new instructor at a technical college in Georgia and have a question about my withholding on my paychecks. I have elected for TRS (Teacher Retirement System of Georgia) and I have noticed that I have zero federal withholdings. They pull out for Fed MED/EE and my state taxes, is this accurate? I don't want to get hit with owing a tax bill at the beginning of the year.