They kind of do have jobs already. You can tell by their Limits.
Squall is a Knight/Paladin
Zell is a Monk/Martial Artist
Irvine and Laguna are sharpshooters (not a standard job, considering most FF job games don't have guns)
Quistis is a Blue Mage
Selphie is a Red Mage as she has several skills that can either damage the oponent or heal the party
Rinoa is tricky, I'd say she's a White Mage/Summoner after she gets the Wing skill, but up until her limits are based on Angelo, so you could say she's a pet trainer (there's a job in X-2 like that, can't remember the name)
Seifer is a Dark Knight
Edea is a Sorceress/ Black Mage
Kiros and Ward are the supporting characters, but if you had to go deeper I'd say Ward is a Dragoon and Kiros maybe a thief because of his blades