A friend of mine was in this situation when he spent a year studying abroad at a high school in the US when he was 16. During his stay, he got a driving licence in Florida (I think), and was officially allowed to use it for 6 months after he returned. Unofficially, he used it a lot longer than that, though I don't know how advisable that would be. For a regular traffic stop, it would not be much of an issue I suppose, but you could get into all sorts of trouble with your insurance company or even the police if you get into an accident (think "driving without a valid licence").
He was able to get the licence transferred when he turned 18 though, which greatly reduced the cost of getting his EU driving licence.