Parenting lesson-how to teach kids to be a leader; not a follower

So my husband and I recently got an email from my 3rd grader’s teacher with the subject line of incident. Great. Just want you want to read on a busy work afternoon. Turns out that my son had been not so nice that day with another student to one of their friends in their class. My heart sank. This does not sound like my kid. But we responded that this wouldn’t be tolerated and we’d have a nice chat with him later on when we got home. Turns out my kid was slightly mean that day, but to no avail, he was lead to do mean things and my son made the wrong choice by following. So my question was how do we convince a 9 year old to be a leader and not a follower. How do you get that point across in 3rd grade terms? Of course we gave him the normal lecture of “put yourself in that kids’ shoes that you teased today-how would that make you feel”, “you don’t have to do everything that so&so does”, “you need to surround yourself with good people and make good choices”, etc…But I still feel like he wasn’t understanding the true point. He’s a good kid, but like all kids, he does make bad choices from time to time. Later on that night we got an email from the mom of the ‘other accused student’ aka my son’s friend. It struck me at that point after reading the email, that this is a bigger issue than just my son making bad choices. He apparently was teaching her son bad words and other inappropriate non-9 year old things. Of course the email read, that by no means were they saintly parents, but they were actively involved in their church, limit their son’s tv/game time, and avoid the radio. So my take on that was, well it has to be your son teaching him these awful things. WHAT?!? Ugh. My husband was furious and started to compose an email that I was not comfortable sending back. So after he edited the section I was not fond of, our message back was simply, I think we need to monitor the kids time together because obviously it’s getting them in trouble. What he (we) really wanted to reply was, “Feel free to go back to your prayers (sorry…I do pray and believe in God, but hate when people throw it in my face) and pray for better friends for your son since obviously my son is the problem-B*&%H!” See…there is an underlying issue going on with my son and her son. They were in the same class in 1st grade and my son got into trouble a lot. They were not in the same class for 2nd grade and we rarely had problems (just the usual, not paying attention issues). Now they are back in the same class and here we go again. I am vowing that my son and her son will not be in the same class the rest of their elementary careers. It’s too much of a coincidence that when these two are in the class, problems arise. My son is by no means an angel, but seems to be pushed in the wrong direction. Again…I ask…how do you all relay the message to your children, that it’s best be to be a leader and not a follower?