I don't understand how someone's life is being threatened by vulgar jokes
Exactly. As long as it's not anything threatening to her life, you're just going to start problems by reporting the incident.
Here are possible outcomes:
You embarrass her in front of her colleagues (some teachers may be embarrassed by offensive comments)
She finds out, there are no repercussions (since it isn't violent) and you start additional problems between her and the students
What do you want to happen? The school to interpret it as hate speech and pursue serious action against the kids, perhaps ruining their future? I see no real upside to tattling on the kids.
(Though, in NO way do I condone the vulgar remarks...)