Lhasa Apso random barking and biting???

Hi all - I so hope that someone here can help me. I have a four year old male Lhasa (neutered). We got him when he was almost one year old. He is a smart dog, and likes kids and cats. Here is where I'm having trouble . . . I run a home daycare hence I have a lot of parents and kids coming and going throughout the day. There are certain parents that the dog has never given a second glance to, and then there are other parents that he barks like crazy at . . . every time he sees them!! Lately, he has even nipped at a couple people's heels hard enough for them to say, "Ouch". Obviously, this is a huge concern for me because I run a business out of my home. I can't figure out why the dog targets certain people . . . it's not just men, not just tall people, not just people with glasses or deep voices . . . it's really random to the point where I feel like I need to lock the dog in the bathroom whenever someone pulls into my driveway.

Does anyone have any input on why this is happening and how I can change the behavior? HELP!!!

Andrea

what I did with my labX Brina is to teach her to "sit,stay" and when parents come in to drop off their children she will sit in the doorway to watch but not come near. (I also have a home daycare) if a parent asks to meet her then I will allow her to come out to meet and greet.