Its a matter of letting go of the pain and hatred of the past and forgetting the bad memories you had before. I'm not a perfect person to say that your being exaggerated for your reaction over your enemy in highschool because I've also been there.

What I just want to say is that as long as it will not cause you any trouble, then so be it. But also, learn how to forgive the person. Its also like forgiving your past lover even though he/she leaves a scar in your whole being. It will just affect your mood and your point of views in life, so its better to let go of your hatred and just think of the good things that happened to you in your highschool years.