This is such a trick issue for some! BTDT! I think the grade skip was instrumental in fixing this. DS says that if you go into a gradeskip with the 'I'm the smartest' attitude, you will fail. Not sure it he sees this as happening to him or not.
I just checked with DS11. He says that many things which are true, just can't be said, if you want other people to want to be with you. He files it under the topic "Life isn't fair."
Here's how I would try to explain why a child 'must not' talk about being smarter than others at school.
Most people grow up feeling badly about themselves. They feel that they aren't attractive enough, smart enough, special enough. This is a giant lie that gets repeated over and over until people think, "Of course that's true." This Giant lie is really bad for everyone, and when it's time to stand up and do what is right, this Giant Lie gets in the way and makes it even harder than it has to be.
Although it is true that you are unusual in having different learning needs than the kids you are near, it doesn't mean that they aren't smart, attractive and special. Every kid is those things in abundance, although if the Giant Lie is piled on deep enough, the kids won't believe it or act like it until later.
Anyway, part of making a better world is personally putting a stop to this Giant Lie wherever you can. It's like that Monster on Star Trek that feeds off of people's bad feelings. I will do whatever I can to make sure that your learning needs are met, and I want you to wait patiently, and not repeat the Giant Lie at school. Some Problems are large enough that they take years to fix, maybe generations! Let's work together to fight this problem, ok? At home, you can tell me all about what makes your mind special, and I will listen and applaud, because I never believed the Giant Lie about my intelligence, so it can't hurt me.
Then you have a basis to look around the world for the Giant Lie. You can point to almost anything bad and trace it back to the Giant Lie. People smoking in public, maybe they were told that smoking would make them 'cool' (attractive) and that they shouldn't think for themselves becuase they weren't smart. Anything that seperates the world into 'Winners' and 'Losers' is part of the Giant Lie.
Anyway, good luck. Our kids do get slammed for telling the truth, and it just isn't fair. But I think that if we can point them towards the bigger picture and inspire them, they will come along.
((hugs))
Grinity
