This didn't happen in high school, it happened in my freshman year of college, first semester, not even halfway through. I know that's not what the OP asked but I remember it often and wanted to write about it anyways because I feel like too few people remember.
I knew a girl who lived down one floor in my building, I'll call her J. I wanted to be friends with her but I was so damn shy, looking back on it, it was so painful being that reserved. We talked some but I never got close to her. Her roommate never showed up for the semester so she was alone but she did befriend one of my other friends, C, and became quite close.
Weeks into the semester, C told us that J had cut herself, taken a picture, and shown C, almost like a cry for help, so C told someone at the school and J was placed into therapy. But being 18, J didn't have to stay and quit after only a few sessions.
One day before heading home for the weekend, C, J, my roommate, and I were all hanging out. J asked if anyone remembered the name of No Doubt's first album so I looked it up for her. She thanked me and me being so shy, I couldn't continue a conversation.
I went home, came back Monday morning, and went about my day like normal. Then sometime in the evening, our RAs came by and told all of us to stay in our rooms. They came for us a bit later and said we had to head straight to the gym for an announcement. As we were walking to the gym, my roommate started panicking and crying: "Oh my god, has anyone seen J? Where is J?" She went from person to person asking if they'd seen her. No one had.
The announcement was that J had taken her own life, probably the night before. C had found her. The door was locked, J didn't answer when she knocked, so security came and unlocked it, never thinking to shield C from what might be inside. This was the first time anything like this had happened in decades at our school; everyone was shocked.
I still feel bad. I know it wasn't my fault and I couldn't have helped but I keep thinking, "What if I had been her friend? Maybe I'd have been around her more." It was said that if she had a roommate, maybe that would have been enough to give her pause, the idea that someone might be walking in literally any second.
As her grandmother made plans to hold a memorial at the school, really disgusting details came out. The school whore had been "friends" with J when the summer session started. J had confided in TSW that she was a lesbian. TSW proceeded to bully J from there on out. She'd "borrow" her stuff, keep it for as long as she wanted, maybe return it, maybe not. J had begun keeping a diary of all the nasty things TSW did to her, apparently too afraid to seek help; we didn't even know TSW was doing this to her. When cops went through J's laptop, they found her diary. Up until then, TSW was such a cunt that she'd been working with J's grandmother on planning the memorial, like she was actually a friend. She adored the attention of it all. When her grandmother was handed J's diary, printed for her to read, she cut the bitch out and word spread that she was a twat.
I would like to think I'm a bit more vigilant now, if I see signs of someone feeling sad I get over my own shyness to talk, just in case. Doesn't mean it will ever make a difference but it's always worth trying.