Called back to the Army..?

My husband served in the Army for 4 years..He was stationed in Korea the whole time..He had just gotten out 2 years ago and now they are calling him back..He is enrolled in school maintaining a 4.0 also we have a 6 month old baby, i really depend on him..What are the chances of him getting an exemption. And if not an exemption, would he get into alot of trouble for ignoring it?