Well, it is true it was stolen. One minuet it was there another it wasn't.
Most of all you will and should get in trouble. You shouldn't leave it laying around.
In school your backpack is like your wallet or to a woman a purse. You keep it with you at all times, don't leave it laying around and keep an eye on it.
Also I think your mom should understand this will happen. When I was last in school in 1992 or 93 my back pack was broken into a bunch of times. I would sit it down in the gym (it was too big for the lockers) & kids would open it stealing paper, pens, 2 expensive calculators & stuff like that. Never stole the whole thing. I finally had to get locks for it to keep on the outside to keep thieves out.
Hopefully your mom will remember how life was in highschool and not put the full blame on you. It true you can't keep an eye on it all the time, but you can be a little better and not leave it outside. You should have taken it with you. ; )