My boys are grown (17 and 24 years) and we experienced a similar thing with our oldest...hearing very graphic stuff from older kids in the neighborhood.
First I commend you for having concern and taking active interest. As a parent you always have choices. You do not have to be at the mercy of the situation.
Secondly, I would start a conversation with the teacher about what is going on and not make him or her feel responsible for it - but simply raise awareness.
Third, I am assuming that these are not your child's friends, or you could be in communication with the other parents of the kids sharing the graphic information. Ideally you would want to be able to talk with those parents too - not to make them feel like lesser parents, but to solicit their support of your "personal dilemma"....asking for help. Who knows, these parents may not realize how much exposure their kids are having to graphic sex and would appreciate the chance to address it with them.
This may sound "drastic" and "bold", but you might want to consider hosting a little gathering of the kids who are involved and include their parents. Once you have a chance to meet and greet, you could follow up later with a conversation about what you have been observing.
The main thing is to avoid "judgemental statements"...and sharing what is going on giving folks a chance to do the right thing without being labeled a "bad parent".
What we have tried to instill in our children is to protect the younger ones, and always respect the restrictions of other parents. If your friend is not allowed to see R movies, and he is 17 years old, don't invite him to attend R rated movies. Find PG13 or some other activity.
If you can establish a "norm" of supporting the ground rules of other parents, it will be easier to talk about the things that go wrong. Kids will always be doing things that need correction. If we are too fearful to connect with their parents and help establish some guideance where it might be needed, then we allow norms of very low standards to flourish, and all the children suffer - as is your child with exposure to images and ideas that threaten her innocence.
Finally, you can choose a different school that perhaps has a better handle on what is going on with the children with respect to virtues, language and relationships.
If you do make some strides working with other parents involved, I would love to write about it in my publication, Banana Moments http://www.bananamoments.com (insights and lessons learned for parenting in the 21st century).