I think that should speaking Good morning once at the day because when we often say "Good morning" we become brazen. The teacher has a lot of other students and each of them said good morning also so teacher can be bored answers. Many people want to be a nice and feels that he should say good morning every time when they see the same teacher but this is not true The teacher will not be offended when you don't say a good day because earlier you said already. You can smile to him or her but not speak often Good morning. So if you said "Good day" already you don't have to say "good day" again.
I think that should speaking Good morning once at the day because when we often say "Good morning" we become brazen. The teacher has a lot of other students and each of them said good morning also so teacher can be bored answers. Many people want to be a nice and feels that he should say good morning every time when they see the same teacher but this is not true The teacher will not be offended when you don't say a good day because earlier you said already. You can smile to him or her but not speak often Good morning. So if you said "Good day" already you don't have to say "good day" again.