I had a Highschool English class once that I caught a 'couple' cheating. I was very young at the time so I remembered all the tricks (though I only cheated on 1 test and felt horrible for it afterwards) and spotted it from across the room. I walked over to the guy and girl and quietly told them, "I need your tests and your book (it was a novel)" and I held out my hand. I tried to be discrete about it and not make a scene but they had the gall to ask why. I said, "I caught you cheating. Please hand me your tests." They denied it but gave me the tests and the book. sure enough she had written the answers in the back cover to about 5 questions. The kid behind them shouted, "busted" and I politely asked him if he done and wanted me to take his test as well. He shut up right away. I left the tests, and a note to the teacher explaining what had happened. They were honor students and both got zeros and a call home to mom and dad. Word got around quick, which was good for me.
Unfortunately since the teacher doesn't seem to care, maybe you could try and just go to the principal yourself with the taken up tests before you leave for the day. I don't know if anything will be done about it, but at least you've tried. You may could say something like, "I didn't know exactly how to handle this since the teacher was not here so I decided to take it to the top and give it to you." This may work if the teacher has never talked to you about what happened when he found out. If anything is brought up negatively you could always say that you weren't for sure if he ever got your note (It IS possible for kids to steal those things so the teacher won't find out.) and that you thought as a teacher he'd want to make sure that someone in charge knew of the situation so it could be handled, and since he wasn't there you thought that that's what he would have wanted you to do. There is truth in this so it's not really a lie. Just because the principal seems slack, he may not tolerate cheating in his school. At least that is what one would think anyway.
But even if you just take up their tests and the teacher does nothing, at least you've still tried and their ignorance won't be on your hands. All you can do is what you know to be right even when others aren't. I think this is very important and that it will be rewarded in the end. Let us know what happens if it happens again.