Mary wrote in asking about aggression in her young female chocolate Lab. She has two other Lab mixes, older and both female. The young one has started baring her teeth and growling at the others, and also has begun acting skiddish and fear-aggressive around strange people particularly at the vet.
Mary, you indeed have a problem that will definitely escalate unless you take some firm action right now. Even if you does you part perfectly, there's still the unknown quantity: the individual personality quirks of the problem dog.
Sometimes you get lucky, and two females get along. Occasionally, three females will get along. But it's not uncommon for females to have much more difficulty living harmoniously with each other than it is for males.
You're taking this dog to training classes now, and I'll bet the dog is performing marvelously for you in that setting. The issue is what happens outside the classroom, when you're not there to give her direction. This Lab needs socialization skills with people AND other dogs.
One way to achieve that is through Doggy Day Care. She may be the top dog at home, but could quickly be knocked down a few notches in Day Care, which wouldn't be a bad thing. She'd learn to play by the pack rules, instead of by her own rules. She'd also go home too tired to pick fights with her housemates.
Another way to socialize her is for you to take her EVERYWHERE you go and literally saturate her with worldly experiences. A Gentle Leader head halter will alleviate the possibility of her biting and/or pulling away in fear. Do not allow people to force themselves on her. Let her come up to them on her own terms.
This dog is probably going through what doggy psychologists would term her "second fear impact period," between 6 and 14 months, when new situations suddenly seem frightening to the dog. She may be combatting her fear by turning bully-aggressive with the dogs she lives with. It's important to teach her FIRM boundaries at home, and to make sure she knows "Leave it!" You can use this command if you see a problem escalating. I would step in IMMEDIATELY from now on if you even SUSPECT she's THINKING about curling her lip at another dog. Basically STOP IT! at that point. Teach her you can read her mind and will not allow her to think certain thoughts!
Don't wait until the fur flies anymore. Often people mistake rough play for dog fights, but now you've got a different situation. Step in instantly if you think she's getting off track.
Make sure she's getting lots of exercise. If you can't walk her at least two brisk miles a day, put her on a treadmill. She has lots of frustration she must get rid of, and a "forced march" is an excellent way to do that.
People often make the mistake of assuming all Labs are docile, sweet, and laid-back. Such is not the case anymore. Probably because of overbreeding, you never know what you're going to get in a Lab unless you're working with an extremely reputable, established, respected, and reliable breeder. There just aren't many of those around. There ARE definitely temperamental Labs out there. But at least you're catching this potential problem at an early stage when you can still do something about it.