How did she make that happen?
Later
Okay, it has been explained to me. Hence, marked “CIDU-solved”.
But it’s already scheduled, and rather than scurry to replace it let’s give some eager cidu-beaver the opportunity to jump in and say “Well clearly, ….”
Comics I Don't Understand 
It’s the artist’s signature.
I’d rather not be considered as a water-logged rodent, but I would say that the action in the strip is easy to spot (hint: look at the corner of the third panel, and compare that to similar corners in other “Nancy” strips). There is, however, one aspect that still qualifies this strip as a CIDU: how on Earth could Nancy legitimately believe that her teacher is going to accept a signature from Olivia Jaimes in place of Fritzi Ritz? (After all, Fritzi is presumably her sole legal guardian.)
P.S. In addition to “meta”, this strip could stand a “4th wall” tag.
“how on Earth could Nancy legitimately believe that her teacher is going to accept a signature from Olivia Jaimes in place of Fritzi Ritz?” Ah, but the teacher needs “an adult”, not necessarily Fritzi (but that wouldn’t work in real life).
Lots of people’s signatures are difficult to read, including Olivia Jaimes’ own signature. Nancy may think that her teacher isn’t going to look at the signature too closely.
@ zbicyclist – Nice catch. A perfectly credible, even if unrealisitic distinction, which is sure to backfire when she turns that form in. Other kids have tried similar stunts before, with approximately the same results:
I remember that, on the show “Welcome Back, Kotter”, Juan Epstein used to turn in excuse notes saying things like, “Please excuse Epstein (!) from homework / class / whatever…signed, Epstein’s Mom”.
Turns out, his mom did refer to Juan by his last name and did sign everything “Epstein’s Mom”