I could only hear the 8 kHz until I turned my volume way up, then I could hear the next higher one.
I can tell you exactly when I lost a big chunk of my hearing. It was about 15 years ago at something they called the "BFD" concert. It was an outdoor rock concert with a whole bunch of artists like The Pretenders and Chris Isaak. I had been to concerts before and knew they were loud, but this was UNBEARABLE. It was so loud, I felt like I was being punched HARD in the chest over and over. It was scary. So, I left my seat and went outside the arena. There was no escape from the noise.
I would have had to go into the parking lot for it to not be painful, but I was with people, and couldn't do that. It was so awful. Everyone else seemed to love it, though. Oh, and yes, I had come prepared with earplugs. It made no difference.
I had tinnitis for a long time after that, and still do sometimes. Thank goodness for subtitles.
I have no desire to hear teenagers' cell phones. And what kind of teacher lets students have their cell phones on in class? Even if you hear your phone squeaking at you, what are you going to do? Answer it in the middle of class? It's not like you can hold your conversation at 600 mHz or whatever. Oh wait, I forgot that people "text". What next? I shudder to imagine.
Surely keeping your hearing, and not being in awful pain, is more important than not offending people?