Dear {School},
I would like to humbly offer an apology to the entire school for my actions on {Date}. I am very sorry that I {Action}, affecting each and every one of you with my thoughtless actions. I know now that what I did was very wrong, and I hope you can all find it in your hearts to forgive me.
I am making amends for this terrible crime by {Restitution}. I am also being punished with {Punishment}. These measures along with apology will hopefully be enough to make up for my crime.
Please accept my humble apology, and rest assured that I will never act in this way ever again.
Sincerely,
{Sender}