Hi DW,
If the schooled ruled in your favour, I think you will find most rational and sane people will too.
Trying to discredit someone over something they said when they were 15 is ridiculous and will seen as just that, ridiculous.
Move on and leave this behind you, if it comes up again you can deal with it in the same way.
You have achieved a great deal in your life since you were 15, and this is what people will see, a competent, intelligent man - not a naive teenager. You have grown up, and it looks like others need to do the same!
Don't worry, you've got this.