Etiquette Question

I went to an early morning spin class at my local gym as the first half of a brick.

A substitute instructor was leading the class. I have sat through a class of his before and nearly fell asleep. The guy basically...sucks. He couldn't raise the heart rate of a meth addict on a three day bender!!!

So my question is: When our schedules are so tight and time is so limited, is it inappropriate to sit in the back of a spin class and just do a tempo ride at your own pace while ignoring the leader's instructions?

I rarely do this, but in this case I felt like I had to or I would be wasting a whole session.

Thoughts?