You are eligible for an in-school deferment if you have a Federal Stafford, Federal SLS, Federal Consolidation, or Federal PLUS loan. There is no time limit for this deferment. You must be enrolled as a half-time student (at least) to qualify for loan deferment.
Contact your lender to request an In-School Deferment form. Once the student is enrolled, the form should be taken to the Registrar's Office for them to complete the form certifying student's enrollment.