Some students are sweet only to manipulate and get what they want. I agree that most kids are genuinely good, and I enjoy interacting with them. I think that there are a lot more situations like that than some. Kids are pretty smart, so they will try to do anything to find the easy way. If they can figure out ways to manipulate teachers, they know that school will be easier for them.
Kids suck up all of the time, or put on a front to get what they want.