you need to trust the people you're training with. you hope they're not sick in the head and want to hurt you for fun. you hope they let go when you tap.
i've been lucky, every person i've trained with has been decent but i wonder what keeps it that way. i bitch about the price but i think it is what keeps the assholes out. if you pay $100/month you must be dedicated to bjj & don't want to fuck up. i don't think the people i train with earn big bucks either so the monthly fee isn't pocket change.
i don't know if the instructor looks out for his students. you have guys walking in from the street getting their free class by learning the armbar on you. the instructor usually has long term contracts with the students, so he'll still get your money if you're out injured by an asshole student.
Maybe getting beat up everyday by higher ranking students makes them quit or adjusts their attitude. Good reason to pair newbies with blue belts & above?
what do you think keeps the assholes out of bjj? or are they kept out?