Midoriya decides to eat at least some of his food. He'll need energy for his escape attempt. If it's laced with something, he'll only feel a little of the effects if he eats just part of his meal. He eats it, not with gusto exactly, but with the economical efficiency of a teenager.

"I'll pretend I didn't like the rest of my food, which isn't hard, and then we'll scout the vents. I'll follow you there in a minute."

He stands, scrapes off his tray, and leaves it with the other ones like a good boy. It pains him to waste food, but he must be careful.