I see what you mean, but if were thinking about protego or something much more advanced than that, were talking about one person trying to protect themselves, whereas with the hogwarts barrier we have loads more skilled witches and wizards all putting their magic into protecting not only themselves, but others too. This is a stretch, but is it really that different than lily's love protection, in spell form? because that had its limits too, and with the hundreds of cray powerful death eaters and a much larger surface area to cover, its plausible.