I mean, he's not wrong about going to mixers. He's not officially going out with anyone and has probably expressed his desire to not get into romantic relationships repeatedly. That outing with Mio was out of returning the favor for her speaking up for him from being accused of groping, with no hints of them going any further than that until perhaps near the end of the chapter.
And yet all the other girls around him are giving him a hard time. I'd only give the teachers a pass because it's a high school boy going to a college-aged group's mixer at an adult-oriented establishment. In fact, I'd have to question Mio's morality for asking a high school boy to fill in as her 'date' to a place and event she wanted no part of.
I'm pretty sure those girls would have to drill into those two first before they grill the MC, but with how kooky this rom-com is, what do I know?