Looks like you're the smock in this scenario. Pro-tip: Dad's old t-shirts -- you kill two birds with one stone.

When I was a kid, I just used one of my dad's old buttoned down shirts--instant smock!

Maybe you could use old bed sheets or pillow cases.

Ahhhh reminds me of the time when my brother was in high school and told my mom they needed to make flan for a Cinco de Mayo class party the day before.

Want to prove a point and ensure they never do this again? Send them to art class wearing (clean) garbage bags.

Yup. Old shirt worn backwards does the truck :-)

This is a teaching moment. They go to school tomorrow without smocks and an explanation as to why. If they pull that stunt again, same treatment. They must learn to tell you in a timely manner when they need something.

Reminds me of the time my little brother waited until 10 o'clock the night before to tell my mom "Oh by the way I volunteered to bring 10 pounds of taco meat for the school party tomorrow!" My mom was PISSED but she couldn't let down an entire class of kids so she made an early morning grocery run and made the taco meat, then grounded my brother for a month for not telling her earlier.

just dig out some old shirts. that's what we used when I was a kid

It's their problem now.