Let me lay out what you actually do the moment something goes wrong, so here is the escalation ladder. Save it. You will not want to be figuring this out while your kid is crying.
Step one, capture evidence before you touch anything
Screenshot the post, the replies, and the timestamp visible on screen. Then screen record a scroll through the thread. Downvoted posts disappear, and once they are gone you are describing something instead of showing it. Store copies in a folder outside the phone, such as cloud storage or emailed to yourself.
Step two, report inside the app
Tap the post, open the menu, choose the report option, and pick the closest category, meaning bullying, threats, or personal information. Do not report from your child’s account only. Report from your own account as well if you have one, since multiple independent reports move a queue faster than one.
Step three, report to the app stores
This one gets skipped constantly, and it has teeth. Apple’s App Store listing has a Report a Problem link, and Google Play listings have a flag option. Store-level complaints about a pattern of unmoderated harassment create pressure that in-app reports do not.
Step four, go to the school
Bring the saved evidence, not a description. Ask specifically whether the conduct meets their disruption standard. Ask for the response in writing. A written record changes how seriously a district treats a follow-up.
Step five, involve police for anything credible
Threats of violence, sexual content involving a minor, or repeated targeting cross from platform issue to crime. Local police can send a preservation letter to the company quickly, which freezes the records before routine deletion.
Step six, look after your kid first
Do not lead with taking the phone away. Confiscation reads as punishment to a teenager who just got hurt, and it teaches them to hide the next incident. Handle the person, then handle the platform.