Teacher Liu, yesterday I joined a Chinese corner. I said "Ni hao" to a teacher, but a Japanese friend told me that was rude. Is that true?

I wouldn't call it rude. "Ni hao" is a very common greeting, and teachers generally won't think you're being impolite.

Then why did she say you shouldn't say "Ni hao" to a teacher? I felt so embarrassed at the time.

She may have felt that students should be a bit more polite with teachers. At school, students usually say "Hello, Teacher." When meeting for the first time, you can also say, "Teacher, nin hao."

So it was just that my wording wasn't natural enough, not that I was being disrespectful.

That's right. And in a relaxed Chinese corner, if the teacher is about the same age as everyone else, simply saying "Ni hao" is perfectly fine too. It really depends on the situation.

Then next time I'll say "Hello, Teacher" first, and after we get to know each other, I'll use a more casual greeting.

That's the safest approach. Don't worry too much because of one person's comment. It's not as serious as you think.