I for one would prefer they never advanced the plot to Yotsuba being in school, or any older. The draw of the manga is the feel-good nature of the thing. How Yotsuba is completely unperturbed by the restrictions and expectations of the world, which we the readers are now subject to. It's to take us back to a simpler time in life before the world had affected us into thinking a certain way, etc.
While I agree that Yotsuba in school and the hijinks that such an event would entail could be pretty entertaining, I feel it might also cause the comic to lose some of the free-spirited exuberance and wonder it has now.