Hi Pamela!
My 5th grade boy began biting his nails last year. I have no advice for you, sorry. It's not an easy habit to overcome, which is why I constantly have my own nails done :o)
I find that the busier he is, with sports and all, the less he bites on them. He does seem to do it more during homework and watching TV. When I visited his class he does it during instruction, as well.
I don't know if there is anything to prevent this. And, in my case, I'm not sure I am going to do anything about it. It's nervous energy (which boys have alot of) and I'm waiting for it to resolve on it's own....which could be never.
We begin the next season of sports next week, and i know he will do "less biting" because of it.
Anyway, I will be reading your respones this morning to see if there is any advice without making my son "insecure" about his hands.
I just wanted you to know that there is another mom out there bothered by this habit :O)
N.