At her age she's heading into adolescence and will 'try it on' with you at every opportunity. As advised above, don't let her 'bite/hold' you at all. See the vid, it's really grand

on how to stop that behavior. She's not aggressive, just a youngster finding her way in this world.
She would be too old for puppy classes, here 16 weeks is the 'age limit'. But yes, take her to a dog school but PLEASE make sure it's POSITIVE TRAINING ONLY and even if they say they use Positive Reinforcement, go and WATCH them at work BEFORE signing up!
I made the HUGE mistake of signing with a school here with assured me they only used Positive Reinforcement (I didn't check first as their new term hadn't started at the time I rang). Two and a half weeks into the lessons, and watching the so called trainer yanking dogs around, she never got her hands on *my* boy, I walked out in the middle of class never to return. I doubt they had any idea what Positive Reinforcement Training meant, they sure didn't use it! Didn't even use treats, let along anything else like a clicker! Believe me though, those two weeks undid all the good training Ra Kismet had had prior to going to the school.
With the barking, all the advice given already is excellent. I always check to see what my boys are barking at, sometimes I cannot see/hear anything, but their hearing is so much better than ours, so sometimes it could be a noise which we cannot hear but they can. I always reassure them it's o.k. I use "friend" for anyone/thing which is harmless and they have barked at, i.e. men on cherry pickers doing something with the over head wires set my Ra Kismet off, I checked, thanked him for letting me know and reassured with 'friend'. I always check what they're barking at, even in the house. Once their bark has been 'acknowledged' I find they stop immediately, it's as if they '
feel they've done their job properly in letting us know there's something they're not too sure about'.
Your girl is young, so use the vids and all the great advice given by other members here and she'll be fine. Youngsters all go through the 'difficult' stage, just like humans
