[Vocabulary] a person who speaks instead of someone else

Hi,

Is there a word, an idiom or a phrase in English which would refer to a person who speaks or explains something instead of someone else who is supposed to answer that question and explain that issue. For instance, if a teacher asks one student why he/she is late, and another student tries to give some explanation or excuse instead of the first student, is there a word or an idiom which would describe a student who interferes although he/she is not supposed to (he/she may have a good intention but his/her behavior is perceived as annoying)?

Thanks in advance!

The teacher might say "You're not XXX's spokesperson/spokesman/spokewoman. Please be quiet and let XXX explain".

In business, an official spokesperson for another person is called a proxy. You cannot use that in a classroom situation of course.