Is the answer a little bit impolite?
NO! (That looks impolite, no?) How about just "No."? (Is that polite enough? No?)
You don't tell us if your teacher is asking this verbally, or in a chatroom in Zoom. Writing "NO!" may be seen as a bit impolite. If the conversation is spoken, then it is your tone and your expression that determines whether it is taken as impolite or not. If you say it nicely, I don't see how it can be impolite. Even a "YES!!" can be impolite, depending on how it is said.
I, however, don't feel this is a common reply:
Teacher: "Do you have any questions in section?"
Student: "No more questions."
I think that reply would work if your teacher asked
Do you have any more questions?
and I feel even then that reply might sound awkward and robotic.
The best thing to do is to focus on your tone and facial expression. If you want to have definitive expressions then the following are some that you can try:
Teacher: "Do you have any questions in this section?"
Option 1: "No, no, I think I understand it now. Thank you!" (or "... I have got it now ...")
Option 2: "No, I think I am good for now. Thank you!"
Option 3: "No, I don't have any questions at the moment. Thanks!"
Again, whether your response is impolite or not depends on how you actually say it. It has very little to do with the actual phrasing of the reply.