I wouldn't rule it out completley, the bag from that school does look similar,
Idk man, my biggest problem with it not being a Montessori logo, was that the blue colour of the bag didn't match the logo that they had on their Facebook profile, however, you just provided me with a bag that has the exact same colour.
g. I'm not sure how Montessori schools work, but if the bags are only loaned then the sibling may potentially have to be actively attending that school when the photo was taken. Then the bag has to be in her(?) car, and instead of moving this semi-rigid plastic bag she decides to sit on it to take her whore photographs. And the flow of her shirt comes down in just the right position to make it look like a pawprint.
In my experience, private schools usually gift these sort of bags during events, like school olympics, participation in some talentshow, a science fair, etc. That's why when I saw the two bags in her car my first thought that the logo likely was of a school.
My biggest problem with it being an animal print is that it's not anatomically correct.

The three top dots are way too straight, and then suddenly there's a fourth dot to the side, none of the animal prints that you can find is like this, it's too asymmetrical, the only footprint that you can find that has this sort of incongruence is human footprints, and even then it's not a match at all neither.
When I saw the Montessori logo, I realized that it fit perfectly with the weird symbol that is behind her.
Now, I'm not saying that you are wrong, or that I'm certainly right, but I will continue to operate as it being the Montessori logo for my search, you can make your own assumptions, and I do not think you are any more wrong than I am, we are doing a lot of speculation with very little information that we have.
It's just my instinct that it's not a footprint.