say that you spent the night with a family member and couldnt go home to change or you spilled something on it on car ride and you asked your friend to borrow something and she only had that
School is almost over for you, so I'd personally stick it out for the last couple days just to avoid getting in trouble. If you haven't gotten in trouble for breaking the dress code before you should be okay though.
Just wear them, I have worn short shorts the last few weeks of school and personally my school doesn't care because its the last few weeks. Even if its the last day all they can do is make you sit in the office, change, or have your parents pick you up. I mean don't wear shorts that your butt hangs out, but I doubt the last day will be that strict on things.
are they excessively short? like booty shorts? you're not gonna get away with that; your school will make someone bring you clothes or give you a detention, assuming that's the punishments. or they'll call your grandma: "your granddaughter needs to dress more appropriate and follow the school dress code!!!" hopefully grandma would retaliate: "SHE WILL DIE OF HEAT STROKE GOODBYE"
go for it dear. Who really care in the last moment