What’s wrong with 我们八点快要上课了？
is equivalent to 要...了


You can say that 我們星期一要上課了(We will attend the class on Monday) or 我們明天要上課了(We will attend the class tomorrow.) but you cannot say that 我們星期一快要上課了 or 我們明天快要上課了。 because when people use 快要, they don't add time to modify somethings in a sentence or component.

In this situation, 要...了 is not about to do something.