Treating somebody poorly, purely for spite or some form of retaliation, seems like a lot of unnecessary energy to expend on somebody who may not deserve that much space in your mind. The golden rule is not about whether someone has the right to be treated better than they have you, it is about discovering grace within yourself and extending it towards all others.
As to why you should, I could use another turn of phrase- “Lead by example”. You had a poor example in how to extend grace to others in the form of your teachers, and you have the opportunity to be a positive example to others in their place.