Batgirl? He wasn't Batgirl. He was Batman. Which was why he had brought her a Catwoman mask, but given the 'Batgirl' comment, Bart hid it behind his back and tried to get rid of it.
"Haha. Yeah. I do look like Batgirl." He said, playing along and quickly pulled off the Batman mask. He'd get rid of that one too when he had the chance.
"Hey look there are some people I know!" He said, pointing out to Conner and Megan and K-K-K-K-Kara. While Mia was looking in the other direction, Bart zipped away to hide the masks in the boys' bathroom and came back with punch for both of them. "Uh. So this is a prom, huh?"