Number 1: I do NOT teach my children to be aggressive! But being ASSERTIVE is a very necessary skill for living a happy and successful life, and it is very ok to assert yourself and let your feelings, needs, and wishes be known!
Number 2: I teach my (just turned 5 today!) son that he needs to look for a way out of things. He can make people laugh (because he's naturally a clown if he feels comfortable, but I've also witnessed him in situations where he's not comfortable, where he acts more sensitive and shy), or he can redirect (yes, I teach that) attention or I guess the normal phrase would be "change the subject". But if someone is not being nice on a normal basis, I want him to walk away. If someone puts their hands on him, he needs to look for an adult to assist, or walk away. But if he feels threatened and "that feeling" in his stomach or throat, then do what he needs to do if there are no adults to help. He is an orange belt in kung fu and we also do a little boxing with him. We talk, practice, role play with him when he mentions that he felt sad or that people were teasing him on the playground. One thing that we've had to teach though is that sometimes children don't know HOW to be friendly and what seems like teasing is them asking to play.
An example of that was when a kid was singing "You can't catch me, you're not fast enough" but I whispered (he WANTS you to chase him) and they had a grand time. Another time, he said a big boy yelled "Hey boy" at him and ran towards him to beat him up, and he ran away. I didn't really like that he "ran away" so we discussed it. I was like "Where was I? How come you told me after we left? Is "Hey boy" all he said? Well...MAYBE he wanted to play and just didn't know your name, I don't know". But the next time we went, I was standing there when the "big boy" came charging up. Joseph was like "Mom! That's him!" and since I had my 1 year old with me, I just used the 1 year old as an excuse and said "Hey guys! Y'all play nicely, but don't squish the baby ok?" The big boy nodded, and Joseph said "You want to play?" and they played for a solid hour. (Midway through, he ran over to me, gave me a fast hug and said "thanks mom" and ran away again). What we're learning to navigate (we meaning my son) is the social cues and when someone wants to be left alone, when someone is being mean, when someone is just wanting to play. (Some children can be hyper, excited, a little bigger, maybe a little awkward, but just wanting to run and play). Trying to show him little ways to figure out what's going on...a new ocean to navigate for us!
However, sometimes, bad things happen and YES a child needs to defend himself, and how to do so! Once we were at a play center and there was a little bounce house type thing and a "big kid" (like 8) was going around pushing ALL the kids down. He pushed Joe and he just said "Hey!" but then he started walking towards my 1 year old. I jumped up and started going towards the entrance, watching everything as I was going around it, when I saw my 4 yr old step in front of the baby and said "Don't touch my brother". The kid stepped around him to push the baby, and Joe stepped in front again and said "Hey! Don't touch my brother". The kid went to hit him and Joseph jumped into his stance that he learned from kung fu and blocked every hit perfectly. PERFECTLY! He had perfect posture, it looked lovely and easy, but the big kid tried to hit him 8 times and was blocked everytime and got mad and walked away. Joseph not only defended himself but his little brother too, without yelling, crying, or hitting. It was PERFECT. (In the meantime, I had stopped to watch and see what would happen because while he was facing the big kid, the baby had cluelessly wandered off and was safe). An employee saw the whole thing and had the big kid go out to his father. I let the boys play, but when he came out on his own, I had a talk with him about how awesome it was, and how proud I was. We talked to his dad and sifu about it as well, which made him happy and built his confidence. I also love martial arts (if you go to a good school with a good, responsible teacher) for this reason: defense, respect for others, self respect and confidence all go hand in hand with martial arts, and I'd suggest it to anyone even remotely interested.