teacher told my sister i was stupid in front of everyone

So I left high school a long time ago. My sister is attending the same school i used to go to. Now on of my ex teachers found out i’m her older sister. Apparently he told her in from of everyone how stupid I was. (I got a D in geography the only GCSE I failed.) She told me she started crying afterward.

Why do you think he would do this? I feel kind of hurt. I mean I know i should’ve passed but it’s not my fault i failed. I tried my best.

this teacher is very unprofessional. he does not seem to have the slightest interest in data protection and that

Honestly, don't worry about it. He's definitely a waste of space teacher, who thinks that he is better than everyone else (spoiler alert he's probably not!).
Also, you and your sister are different people, so make sure she doesn't cry when you are brought up ...