Do you get this? "I'm not going to lie, I haven't started the paper yet." "I won't lie, I've been procrastinating a lot." In emails, I mean--I can't recall having a student say this to me in person (though it may have happened). Why point out that they're telling the truth? I generally assume that they're telling the truth (although I'm not so naive as to believe that students always tell me the truth; rather, in most individual cases, I assume that a student isn't lying unless I have a good reason to believe that he or she is. It doesn't make a difference, in most cases: You do the work or you don't, whatever the excuse). In fact, in these cases, pointing out the truth-status of one's claim immediately makes it--or preceding claims--suspect.
So again: Why inform me of the fact that you're not going to lie? Because here's what that does: It leads me to assume that, in other cases, you have lied. Furthermore, "I'm not going to lie: I didn't do the homework" doesn't get you out of doing the homework. There are no points for honesty here. Am I supposed to feel somehow privileged that I'm the one professor whom you choose not to deceive? Are you to be congratulated for your supposedly exceptional ethical sense, which somehow mitigates your laziness?
I know, I know: "I'm not gonna lie" has become a Phrase, a Thing, People Say It. (I find it irritating, to be honest. In my curmudgeonliest moments, I mentally compile a list of New Things People Say that annoy me. "Remodel" as a noun is right up there; at least I normally don't encounter it in my work. So is "speaking to" an issue. Oh, there are so many; a parenthetical can't contain them all.) So it probably doesn't mean much, in itself, except as a sort of awkward transition into an admission that they're somewhat hesitant to make. But in the last six months or so I seem to be getting it a LOT (further confirming its status as a Thing), and I never know how to respond.
So I don't respond--to that phrase, anyway; to the emails, it depends--and I'm using this venue to say the things that I'd like to say. And if you'd like the more concise version, below are three slightly pithier rejoinders from which to choose.
a) I'm not going to lie: calling attention to the truth-status of your claims is a weak rhetorical move.
b) I'm not going to lie: you're failing the class.
c) I'm not going to lie: I still think you're lying.