Is my dog aggresive....help!?

ok...so we just got this puppy (now 4 months) she is part lab. We got her from a shelter...she is nice jumps and chews (but all puppys do) but she always seems to bite my son's (13) shoes...well this is progressing...she now jumps up and mouths nips (not sure exactly what to call it) but it is enough to make a scratch and draw blood. me or my wife can come out and tell her no and she will run off...she wouldn't dare try this on me or my wife we have only had her maybe a month..im afraid she may be aggressive! Is there a way to stop this, or should we take her to obedience class or find a behaviorist or professional....