You have to start repaying. You can take up to a 6-month grace period, but if you do this, I don't think you can take the grace period when you graduate.
When you start school again, you will be classified as a student again and will no longer be in repayment.
Also, remember that your interest rates increase when you are in repayment.
You *may* qualify for forbearance and/or economic hardship during your leave of absence, but I cannot guarantee it. You really should speak with your financial aid office and call your lenders! Only they can tell you an accurate answer.