What to do with an aggressive puppy?

Hii, um so I recently got a pomchi (pomeranian and chihuahua puppy) which I named Emilie on February from a family that was selling it, so I am not the first owner. When I got her, she is so sweet and adorable but the problem is that she begins to bark like crazy and won't let anyone touch her when guests come by. She literally went crazy when the doctors just wanted to give her vaccines and the doctors told me that this isnt normal puppy habits like puppies should be pretty friendly. I have a feeling she had some pretty bad experiences with the previous owners because when I went to pick her up the baby girls were yanking her tail and playing very rough with her ( they were too little so its not their fault but my puppy still must have endured some hardships if shes this frightened of a person just wanting to pet her.) So the problem is that when I take her out on walks everyone wants to come and pet her because she is so cute and tiny, especially the little ones yet I always have to tell them that she is too aggressive to be petted. My brother's friends also all hate her, telling her to *** off and stuff cuz she can't stop barking. I want her to get comfortable and stop being so aggressive so I took her to dog school, but that isnt helping because I am in a class with other dogs and she is too frightened to be with the dog trainer and the dog trainer herself is too scared. Wow I am rambling too much... well so basically, is there a way to stop her aggressive behavior when she is now 7 months old, does anyone have someone they could refer me to or some tips. Thanks soo much.