worked in hearing conservation for a few years. Not sure what you're asking but 80 dBA for 8 hours gives some people a hearing loss. That's the same as 85 dBA for 4 hours or 90 dBA (A-weighted) for 2 hours.

THe fact as an instructor you'll also be yelling OVER the music won't help either. So put the speakers AWAY from you and don't turn them up too loud.
Some of your students don't like the music that loud anyway.