CJane, kittens are pretty much litter box trained when you get them. They take to it WAY early... (that's why they're SO much easier than puppies).
I had this problem last year with my then 13 yr old dd last year. She committed to being a Jr. Counselor at camp & was gone during the week & home on the weekend, every other week. She brought home her kitten from the barn (where she rides, where summer camp is) and then decided she wanted to stay home with her kitten.
I just told her that her kitten would be there when she got back, and that she had made a commitment to work and she has a responsibility to follow through on it.
She brought one home again this year, then left him for me to take care of. lol
I was SO proud of her though -- She's a full-on counselor this year, no more Jr. anything, and working 3 weeks, then off one. On the way back to camp Sunday, she was complaining about one of the other counselors who tends to blow them all off when something else comes up. She said "You know, mom, we made a committment to work and we have a responsibility to follow through, and it's just so wrong that she doesn't show up."