Hopefully the OP will explain the situation a little better than above and honesty is always the best way to go. The OP did say "the screen was cracked". A cracked screen in my opinion would mean the iPad was subjected to some sort of "significant abuse".
The OP says it was in a bag under a seat? What seat? A car seat? Did pressure from sitting in the car seat crack the glass? Was it a seat in a classroom? Did the person behind you kick it or step on it? Did anyone have access to the iPad while it was under the seat? What else was in the bag? The devil is in the details. Glass generally tends not to spontaneously crack.
From what I can tell from the photo the crack appears to be all the way across the left 1/5th of the screen. That would suggest some sort of excessive pressure on the screen at that position.
I predict Apple will claim physical abuse (doesn't matter if you don't know how the physical abuse occurred) and not replace it under warranty.
If the iPad was bought with a credit card the recommendation to check with the credit card company to see if they have 90 day accident protection for items purchased with their card is your best bet in my opinion.
Do come back and let us know how things go.