I would say to drop them off in the morning. Let the center hand them out as the teacher gets a break, so she can open it and enjoy the excitement of getting a surprise WHEN SHE HAS TIME. It is after all, her place of employment, and she can't just stop to open a gift from 10-15 different parents and still give the kids the care they need.
Imagine if all the parents dropped gifts off say, the last two mornings before holidays.......... The kids may wonder .... why didn't I get one, how come my teacher keeps leaving class? What if one of the parents can't do a gift for the teachers- all of them. They may feel like they have to if they see other parents making a drop off to specific teachers. If your center has caring teachers, and they sound like they do..... I think they would appreciate the dropoff plan, and they will be able to give you a proper thank you, when they see you next.