Tell her you've actually got to go for Lunch after all

"Wait!" You say, patting at your pockets because that is something people do when they've forgotten something (which is the sense you're trying to emulate). "I forgot! I need- I've actually got to go out for Lunch after all. I'm sorry!"

It's the best you can do under the circumstances.

"Oh. Okay." Lexi says, shoulders drooping a little bit. But then, too quickly for you to start to feel bad, she perks back up again. "That's no problem! I can do one of the more boring ones for now, since I've got to get 'em done eventually anyway. Or, to be honest, it's more that I want to get 'em done eventually."

You're glad she's being so understanding, even if she's probably a bit bummed out.

You don't have to choose between the Sorority and Lexi if you just leave them to do their own things, regarding each other. And if that's ever not an option, then you can make a choice then -- for right now, you'd like to try keep things friendly on both sides. You could've gotten that done by lying to Lexi and saying that you didn't know who they were, but you don't want to trick her like that, it would feel like manipulation....

Lexi leads you back through the vents to the computer room hatch, since you can't navigate these vents on your own, and lets you out. You can tell she's a bit down from the sudden switch-up from being available to not being available, but overall, she's still cheery and chatty all the way to the hatch and no less so as you say your temporary goodbyes. So you shouldn't feel too bad -- it looks like you've not hurt her feelings in any real or lasting way.

But hey, on the plus side -- hanging out with her and eating snacks just now wasn't much of a time investment, so even off the back of that, you've basically got a full lunchtime free to do whatever you like with. The day is yours! You're also not hungry anymore, and were lucky enough to get full up on exclusively yummy stuff. So overall you're doing well, despite it all.