@Keasaer Hi Keasaer, translator here. I don't think you're missing anything. The original does specify "being in a room together", but when I couldn't find a way to fit it nicely into the sentence at the time, I slashed it hoping context would be enough to imply what he meant(woops). I've since updated it to, "This is the first time it’s ever been just me and a girl from my class all alone in a room like this, so...". Does that make more sense?
Regardless, Thanks for speaking up about your confusion, and don't be afraid to @/PM me if something doesn't make sense.