L.J.
I would talk about how she's a big girl now, since she goes to school. Then a little while later (a day or two) I would mention that she doesn't need the jelly beans anymore and I will stop them in one week. Make it a countdown. Six days, five. . .Talk to her about what other kind of big girl treats she would like for a whole week of doing well (like a Happy Meal or a video rental).
Whatever the subject is, I've always found it best to give my kids advance warning and a countdown. And stay with the stickers, so you and the school are on the same page. I did stickers with my kids, too. It worked!