first post, need some tips

Is she intact - ie, not spayed?  She's matured into her adulthood at this point, and with it, she may be telling these other dogs to back off of her toy because her preference is for the toy play more than them.  So be it.  I go to the park for a walk, but I'm not so much interested in meeting strangers who come up to me, either.  Almost every single Schnauzer I see at dog parks exhibits the same thing you seem to describe - the dog barking at any dog coming up to them b/c they are far more interested in playing w/the toy (ball drive) than the other dogs.  You have a breed with certain preferences - and playing with other dogs may not be one of them.

You'll need an understanding of what is going on with her before you start telling her no.  If she feels threatened by the dog and you tell her No, don't defend yourself, she's learning she can't trust your judgement of who is a threat and who isn't.  If she's resource guarding her toy, then forcing her into the situation repeatedly without dealing w/her resource guarding outside of the dog park is unfair.  Like bringing a kid to yet another birthday party to tell him "no, the presents are NOT for you." again and again.  

Things to consider - more info needed, but I know others will be in to ask questions and help you suss out some answers. 

Welcome to idog.  I agree, your girl is maturing and she simply doesn't want to play with others.  Plus, taking the toy gives her something to resource guard.  The older *I* get, the less inclined I am to play well with strangers......I think it's time to find another activity that she enjoys more and doesn't feel the constant need to protect herself. 

Jack's are typically not great with strange dogs.  Her behavior is normal for her breed and considering the situation.  Dog parks aren't a great place to take a dog like this, IMO.

 Wow, thanks for all the responses already.  I don't know a ton about Jacks, but I do know that they are very opinionated, for lack of a better word, by nature.  We have always urged her to interact with other dogs from a very young age, but from your responses, maybe we will have to find other activities for her.  When we first got her, the breeder we got her from had suggested that we get her involved in obstacle course-type activities.  Perhaps that's something we should look into.

In response to your question, Miranadobe, she is fixed and has been since she was about 4 months old I'd say.  

 Obviously I can't get in Bailey's head when she nips and yelps at the other dogs that want to smell her butt and say hi, but it seems to me that she's saying, 'hey, I'm not interested in you, take a hike' from her body language.  And if you believe that that's how she's wired, then there's nothing we can do about it.  There's a dog down the street from us that she has gotten to know a bit, a larger dog.  And they seem to have fun together, she's not defensive around that dog.

The way I feel is a bit disappointed, because I enjoy going to the dog park and interacting with the other dogs and owners, its just that Bailey only enjoys the playing with humans part of it.  But what is in her best interest is not going to the dog park anymore, then that's the way it'll have to be I guess!

Lots of good info about the breed at terrier.com. 

Ryno61285e

  And they seem to have fun together

Maybe, a play date with a few dogs that she interacts well with, is more her style. Tootsie, my Corgi is very much like your Bailey when it comes to dog parks. Anyone that is rude or invades her space she quickly tells to back off and DOG forbid someone tries to hump her. She is a people dog not a dog 's dog.

tiffy

Maybe, a play date with a few dogs that she interacts well with, is more her style.

Great advice.  It's always a crap shoot at the dog park, IMO.  One day you may encounter a dog who reacts badly to Bailey's reaction and if Bailey is like most JRT's she won't back down from a challenge.  It can end badly for everyone involved.  

 I agree with the others on seeing if you can find play dates with one or two other dogs of similar play style.  I took my girl to the dog park a handful of times.  She's super dog friendly, but the stress of that many dogs around her caused her to start giving warning growls left and right to dogs who got too into her space.  She doesn't like dogs who jam their noses up her rear end and so while she wouldn't snap at them, she would growl and air snap if they didn't back off.

Now we meet up with some friends' dogs on occasion and she has a great time with them.  She's much more relaxed, is willing to wrestle and play bitey face with them.  The last time we got together with a friend and her three dogs I didn't hear one warning growl from her. It was wonderful to see her play so well!

I've also met up with a group of people off meetup.com who all have great dog friendly dogs.  Usually it's just a handful of people who get together so it's also a great situation for her.

And I highly recommend agility or flyball!  Jack Russells tend to excel in such areas and your dog might love it!