If you send a message to others including Kevin then just send one short word or a short sentence like: Yes, no, Ok, later, or “I have class now.”

Please don't send messages to others during class.

If Kevin sends you a message on Skype, you don't need to answer immediately if you have a class.

Kevin doesn’t expect you to answer immediately.

Exception: The student is here.

(If Kevin asks you if you are talking to the student or you previously told Kevin the student was offline)

Please be careful, sometimes teachers do something else in class unconsciously if the class is boring: reading other books, sending messages to someone else, or downloading files on the Internet. It would be better "not doing anything in class" than "doing something else in class."

If the teacher is getting distracted in class once in a while it can later form a bad habit.

If you have to type something to Kevin or look up a website for your class, explain what you are doing to the student. Often the parent is watching the class, and this way they will understand what you are doing and why you might be typing outside of the conversation with your student.