I suppose that depends on the country. The Berlin Philharmonic Orchestra was exempted from Military service in WWII. In the US you could be exempted for various reason, members of the US House and Senate were exempted from service, exemptions were granted to conscientious objectors, physical, mental or psychological reasons, ministers were exempted as were divinity students, exemptions were granted for certain occupation like miners, farmers, mariners and railway workers. You could obtain an exemption if you could prove to the Draft Board that it would be a hardship on your family and children or parents or if you were employed in some position that was essential to the government's needs. You could get a deferment to continue your education. All of these were subject to later review and could be changed at the whim of the Draft Board.