I am developing a web application where several clients can send messages to each other, it would be like instant messaging without strong real-time constraint.

To illustrate the situation, let's imagine a classroom where students are on a web page and can send messages to their teacher who is on another web page displaying all the messages. The teacher can answer questions and therefore the pages of the students who have received answers must update with the respective answer.

The first research of solution are unsuccessful because it turns around form and installation of specific library.

What is the easiest solution to implement? For information, I am developing with javascript, php and mysql; I have no possibility to modify the server part nor to install libraries.

Thank you. LG