In the enclosed space of the dry cave-in, there's an echoing sound of dripping water as if in a large chamber with a puddle.
Hugo's lighter goes out yet there is still light in the cave-in pocket.
According to dialogue, Hugo applies a tourniquet to Rebeca's bleeding leg. A true tourniquet cuts off blood flow entirely (which is why the Red Cross doesn't teach this anymore). Since it takes all day to dig Rebeca out (at least quite a long while), her leg should be long dead from lack of blood and in need of amputation.
Trapped in an enclosed space, Hugo keeps his lighter burning rather than conserve oxygen.
Word is that Curro was seen nosing about the school, picking up the airborne virus before the quarantine went into effect. No previous footage bares this out, making it seem a story point devised on the spot.