If the question was "Would you turn in an entire class for cheating, if they would know it was you who turned them in?", I would probably have to elaborate an answer but ultimately say no.
Of course, I would admit in the beginning that the morally acceptable thing to do would be to turn them in. But then I would go on about how it would ruin my social life, etc.
Bad answer?
Look at the BIG picture. It's not about YOU. As much as I hate to say and act as such, viewing this situation from how it will affect you will make you look terrible.
Back when I was a little nub in high school, I was asked the following question for enrollment into the newspaper. I answered the question wrong.
"If you had a newspaper assignment due tomorrow, as well as a chemistry exam you needed to study for, only time for one, which one would you do?"
Being an idiot, I voted for number two. This is the WRONG answer in all cases from a moral standpoint. Number 2, you let your entire newspaper down. Number 1, the only person you let down is yourself.
You need to look at it from the community perspective. On whichever level of the community you look at, you will notice that different answers are right. Your answer IS acceptable in one case that I have thought up in terms of not telling. However, it will not fly based on your reasoning.
Community (Your class)
You do not turn them in. While it is morally responsible to do so, by turning in your entire class, you place severe mistrust amongst your peers. You ruin the entire class's opportunities in life. They may never cheat as group again, but they will never consider themselves part of the group either. Such a level of mistrust is dangerous, and with such a large amount of collaborative cheating, you place a large amount of scrutiny on any member of your class for the rest of their academic careers. From a class-community standpoint, allowing them to get off develops stronger bonds, ensures that they trust each other, and does not directly benefit any member of the community more than any other. All have equally gained in advantage, and the playing field is still even.
Community (Your university)
You turn them in. It is morally responsible to do so. Your class does not deserve any more advantage than any other class that has occurred or will occur. While your class may not trust you, your university will have a positive example for which to demonstrate the importance of integrity. It will also provide other classes with similar circumstance motivation to come forwards and admit to have done so. It increases the overall university's awareness of academic integrity, and despite the minor embarrassment of a single class being caught, will be noted for any effective punishment it places on the class. It also increases student/professor bonds and allows your class to at least receive some sort of community help from the university to prevent them from ever cheating again.
These answers took a bit of thinking to write up. I'd probably ramble on for about 5 to 10 minutes if asked this at an interview, given how much I've analyzed decisions in the past.