we don't actually see(read) any dialogue/conversation between them. Youta just sees Izumi and Reiya in the same place, and a notebook is being passed between them.
Immediately after a gaggle of schoolgirls trample Youta because they spotted Reiya, and then Izumi appears before Youta, offering him her "favourite handkerchief" to wipe the comedy grime off from being stampeded.
and when Youta asks "why aren't you swarming Rei?", she responds "because you were here", and thus the series kicks off.
But they actually did interact with one another at the very start of the series. It's just that the context of Youta and Izumi getting closer to one another was absent, which is the focus through which Reiya approaches her here and now (after spying his brother leaving school with Izumi from an upper story classroom which clued Rei in to Houjou's importance in Youta's life (with Hasegawa present with him, funnily enough)).