The question "Is white a color?" is not a well formed physics question;
it is not a physics question at all. Thus I don't risk forfeiting my
amateur status by answering it.
As has been pointed out by more than one person already, there are two
answers to this question. One of them (No), however, is incorrect.
White is any one of many discernibly different colors having that name.
To demonstrate this to an eighth grader the teacher should first equip
herself with two or three sheets of differently colored white paper.
She should then ask the student what color each paper is, presenting
them one at a time. It will then be easy to demonstrate that they
differ in color from one another.