What can I do about the constant interruptions from the office and children sent from other classes with messages?
I agree, having a mailbox set up would be a good idea, however, I am more worried about why you keep getting interrupted. I would say, if it is constantly being done by one teacher in particular, I would just talk to that teacher and ask that if it is an emergency then interrupting is okay, but if it is a simple question that the teacher can ask during specials or downtime, then please save it for then.
I completely agree with you.