Probably one 30 minute session in the eighth grade is typical, customary, planned into the syllabus, etc., to provide information about the senior trip.
But apparently the slide show didn't accomplish that-- since it was all about a trip they weren't going on. Won't they need another 30 minute session devoted to the actual trip???
In a different part of the room helping or working with one or more students with the understandable expectation that the other teens should be trusted to behave? Out of the empty classroom where the door is required to be kept open? Briefly talking to another teacher in the hall? Answering an emergency call of nature? How many of those emergency calls of nature leave kids unattended? In the 27 years I've been in front of the classroom-- even while pregnant-- I've never, ever left a class unattended. We don't hang out in the hall talking to each other. And maybe it's just my school, but our rooms aren't so very large that I can't see a student who is out of his or her seat, up at my desk. As a math teacher, I'm frequently at the desk of a kid who needs a bit of extra help. But I can see every single kid in the room, particularly one who is standing at my desk. Nor do I leave anything "snoop-worthy" on my desk. So if a kid does glance down on his way to the door to use the bathroom, he's not going to see anything that would cause me any concern. No need to retaliate.
Those are all guesses, I truly don't know.