Son skipping school and someone wrote faggot on our window

I found put from my sons teachers that he's only gone to school the first week and skipped school the other 2 weeks. I asked why he's been skipping school and he didn't give me an answer. At first I thought it had something to do with my son having a falling out with his best friend last month. Idk what happened but he came home in tears saying they aren't friends anymore.

This morning my son opened his window. His room is right by the living room and his window is at the front of the house and someone wrote Faggot on his window and we don't know who. He still refuses to tell us anything and my daughter (his twin) says she's been getting bullied at school because people think her brother is gay. One girl said "I heard your brother spends a lot of time on his knees" and laughed at her. I can't Gey him to talk about what happened between him and his best friend or what's going on at school. How am I supposed to help?

Edit: He's 15 and they don't really call home if your child doesn't show up to class.