Struggling with my teen’s bad behavior at school

So, my child is in 8th grade, and I frequently get messages from the school about bad behavior—disrupting class, being rude, not listening to teachers, and so on. Their dad only sees them during holidays since he lives across the country, so all the complaints come to me.

This has been an ongoing issue. We had them evaluated for ADHD, but nothing was found, and our application to BUP was denied. We’ve also worked with the Family Welfare Office. We’ve tried giving consequences, having deep conversations, and offering support, but nothing seems to work.

For context, I have two other kids in school who behave well. They’ve been raised the same way, so I can’t figure out where things went wrong. My 8th grader has friends, hobbies, and is challenging at home, but manageable. They’re polite when visiting others but won’t talk to me about what’s going on and refuse to open up to anyone else.

I can live with a bad grade if they’ve tried their best, but I expect them to treat people with respect. Is this just typical bad behavior, or could there be something deeper going on? What else can I do? I’m at a loss here.