Confused on how to make an ER diagram where I have a STUDENT and a TEACHER, and these users have access to a message system where they are able to send and retrieve messages from each other.

A new message thread can be created by either the TEACHER or STUDENT if and only if they have previously communicated with them on another thread. In this thread, the message has a direction where it either goes from TEACHER to STUDENT or STUDENT to TEACHER.

Update:

1. yes only one student and one teacher can communicate in a thread with eachother.

2. either of them can start a message thread with eachother.

*I realize "new message thread can be created by either the TEACHER or STUDENT if and only if they have previously communicated with them on another thread." is contradicting so just omit it.