No pants

"Hey Mr. J!" says the friendly voice of one of my female students (10th grade) in the hallway. I look over to reply, and realize she's not wearing pants.

"Hi, oh, er, ugh, hi" (look away, fumble with keys, open classroom, escape inside).

I'm not really sure how to respond to this situation. It happens a lot. There is much less of a concern about prudishness of all sorts here (nudity, drinking, sex, etc). There aren't really very good (any?) locker rooms at the school, and the students do real gym class here (Stoyan makes 'em run!). So after gym, they change back into their school clothes. In the hallway. Next to my classroom.
A few students try to be discreet, pulling over a sweater while they change their shirt, or turning their backs to the hallway, half-way hiding inside the locker. These students are the exceptions. Most are chatting with each other (boys and girls) and whoever walks by (like me) as they change, down to bras and underwear (the hallway is often COLD this time of year, so they don't exactly linger).
After teaching at an all-girls school in the litigation-heaven of California, I got very sensitive about not putting myself in compromising situations. I tried hard to never be alone with a student, never say or do or look any way that might possibly be construed the wrong way. When you work with kids, once you lose your reputation (whether accusations are ever proven or not), your career is over. So it has been a difficult transition here where lawsuits are much less common and personal boundaries apparently very different.
I don't want to be rude by not replying to the friendly student, or not making eye contact (it's also sometimes a guy in his underwear...I get uncomfortable either way), but also don't want to accidentally look too long or really carry on a conversation with a half-clothed minor. Yikes.
Suggestions?