You have a phone, simply record your interactions. Put it in your pocket and when you are out of ear shot, let it record. Sooner than later you will have the proof. It's very hard to argue with that and if the teachers try to ignore it, send it to a newspaper by telling them all about it and how the school does nothing. Or you can even go the route of social media. TikTok would LOVE this. You can always spin it as you making social media to cope with whatever happened but that you are getting bullied by that person. If and should you need support, you will have that. That does include risk exposure however, so keep it as a last ditch resolve.