One of the most important things I learned in college was to say "I don't know". Many students (even college students), will dance around a question trying to get a hint of the answer. I have no problems saying that I don't know, followed by asking the teacher what was the answer/solution.

Teachers are much more assertive when you admit that you don't know, rather than being trying to prove your worth everytime.

Of course, this kind of maturity is hard for little kids, specially when the "differences in power" between the kid and the teacher are the biggest.