Two comments:
First, if you are an employee of the studio, you likely have no rights whatsoever to the photos that you take -- any rights that might accrue to a photographer probably reside, depending on the labour and intellectual property laws in your country or state, with your employer. So quite apart from problems with the subjects of your photos, you might be offside your employment contract if you use photos you take on the job in your portfolio -- best to get clearance (which I would think your boss would grant).
Second (and I know I say this at the risk of eliciting a series of NONONOs
) I don't agree with jdepould, for two reasons. First of all, a portrait session like you are describing is a commercial transaction governed by contract (which can be oral or written). The question of personal use of a photo by a photographer would be resolved by trying to figure out what explicit or implied terms in the contract speak to it. I can definitely see a court holding that it is an implied term of the deal that a person hiring a photographer to do a portrait is entitled to control all uses of the photo. Second, I think that even apart from worrying about the contractual aspects, there are limits on the use of a photograph. Use that would tend to demean or embarrass a subject (sharing a nude photo on the web, or using the photo in association with hate literature, etc) would likely be considered an actionable invasion of privacy even absent commercial exploitation. I don't think it is stretching things too much to imagine that the subject of a photo taken in a private setting (ie not in a mall or at a football game) would have some expectation that the photo would be kept private. Privacy law is a moving target, and it is definitely moving in the direction of more protection not less.
EL