The group did not do it as a juvenile joke. The 16 year old girl very likely did not do it as a juvenile joke. I don't believe that this is a very crucial point at all.
What about the other side- do you think the teacher's and faculty of this school oppose breast cancer awareness?
My understanding of what happened is that some kids were tittering in the classroom to the point that teachers and students found it annoying-- not because they were opposed to the original message, but because it became a sexual joke.
Again, if this happened at my place of employment, there is a process in place to stop it. My employer has a legal responsibility to provide a workplace that is free from sexual harassment. The school has a similar responsibility.
The point is that you can have a serious discussion about breast cancer as a serious issue without this.