Why are they reacting like this? How can I diffuse this situation?

I think anyone who works in a school has a type of responsibility to report/respond to someone in this type of situation. I definitely do not envy your position. If I were you I would have a meeting immediately with the teacher and principal stating as firmly as possible that you were not ever molested/raped. Make sure you explain clearly why you chose to write the poem on this topic. I can understand why your mother was upset; I'm sure she's facing a lot of scrutiny due to this. Just let every party involved know that this was purely artistic expression and you in no way meant to imply or accuse anyone of anything. I'm sure it will all get worked out and the coast will clear in no time. Good luck.