Really can't have nice things, can they.
That object at the end looks like a cake/bakery box--it's crumpled up, but on (our) right side it looks like the top, where two flaps would join and close to form a handle and slot through slits in the two adjoining sides of the box to hold it all closed.
That, and the "dark liquid-y substance" would be jam or some other filling; if it were blood
(assuming head wound), there'd be a lot more (even small cuts to the head bleed a ton), and there'd be obvious blood around her head as well (again, because of tons of bleeding from head wounds).
I'm guessing she's overworked due to being a single parent and has taken on too much because they're struggling financially. She wasn't eating with them, meaning she's forgoing her own meals to ensure her daughters get enough to eat, themselves. And going out early/staying out later means likely more than one job, and she's exhausted herself.
So in all likelihood this wasn't a home invasion attack, but overworking to the point of collapse is not something trivial. And it's still an issue because her daughters, and our ML, will be stressed out and feel responsible for not having seen the signs, and for feeling like they might have pushed their mother to this point out of a sense of obligation, or something.
So the trick will be how they solve the (presumed) financial strain, in a way that high schoolers can manage.