Well, she's 8 and I think this was a perfect time for her to try this and a perfect time for her to get caught.
She tried it, she got busted....she will get busted every single time. Did she really think you wouldn't find out? Did she really think the teacher couldn't tell you didn't sign the initials? She was hoping to get away with something and whoopsie....she didn't.
I know a ton of kids who have tried this. It's not so much a matter of lying as it is a matter of them hoping you won't know.
She tried. She failed.
I would ground her, not for life or anything, I would talk to the teacher and let your daughter know that from now on, anytime something is sent home, you will be notified by e-mail so no use trying to hide anything because you'll be looking for it. Let her know you're disappointed, mainly because of the trust issue, but also because she apparently thinks the adults in her world wouldn't figure it out.
Mom always finds out.
You know, my son was dawdling about getting ready for school one morning and I could hear him in the bathroom. He came out with the thermometer and said I better take a look. He said he was sure, according to the termomometer, he was sick. I took one look and said, "Son, you held it under the hot water a little too long."
He got this shocked look on his face and asked me how I knew.
I said, "Because according to this, you are technically dead. Better luck next time. Go get ready for school."
That's all I needed to say.
He tried it, got caught. (A boy in his class told him he got out of school by putting it on a lightbulb, but my son thought that was too dangerous).
Kids try things.
They find out it doesn't work.
You move on from there.
Just my opinion and best wishes.