Sometimes when a sufferer looks fine and is simply staring straight ahead- they are dissociated and acutely aware of what is going on around them. (Meaning that they realize they're in a public place, but they're still dissociated and they know it'd be weird to draw attention to themselves for not knowing what's going on). If the sufferer has that far off stare, as if they're daydreaming or in a trance, ask them if they are okay, and tell them what they were just doing. Often they won't be able to hear voices that aren't specifically directed at them during this time; so make sure to direct it to them. Example "Jen? Are you okay? Listen to the teacher. We're learning about blank and we're at blank." or "Did you hear the last bit? We're doing blank at blank."
Sometimes things tend to "morph" for me during dissociation/flashbacks. (People "morph", Rooms "morph.) Example: My best friend now is sitting in the same position my best friend then was sitting in during the trauma, she turns into my trauma-best friend. My classroom now turns into my classroom then. I'm hearing two lectures go on at once. So, if you talk to them, and they look blank or confused DO introduce yourself. "My name is blank. I met you when blank."
It saves us a lot of embarrassment over having to uncomfortably sit through a conversation with someone we sort of know but don't. When we realize that these people bring on flashbacks and dissociation, to have you understand without us saying so also releases the fear that you'll leave us or think we're crazy.