Pam, it's not that unusual and it's not that 'wrong' for this to happen. It's OK, he's not necessarily the major and total manipulator you fear. The answer is more complex and yet more simple.
What you describe is VERY common. What we often forget with a child who has a diagnosis of ADHD or similar, is that they do have some control over their own actions. The amount of control is on a sliding scale from none to almost normal. That scale shifts and changes as they get older and according to the social situation they happen to be in at the time. Other factors are how tired they are; how mentally exhausted (worn out/burned out) they are; how frustrated they are; how safe they feel. Ironically, ther safer they feel, the more likely they are to let a meltdown happen in full. Sometimes when they're just about at the end of their tether, or it's been a bad day, these kids are far more likely to let loose, especially at home where they know they are loved unconditionally.
Your child was safely at home. But ten you brought a stranger into the picture, in the form of the doctor (via the videotape). You added one very strong controlling over-ride, and it was enough for him to pull himself together.
But they can't hold it together all the time. So they hold it together where it is most important for them to do so.
Think about the average man of the house going out to work. Think about all the old "Blondie" comics you may have seen over the years (I know, I'm showing my age). Dagwood at work is a different person to Dagwood at home. At work he has to take a lot of crud from Mr Dithers, which he wouldn't take form his son Alexander at home. Or his neighbour Herb. At home Dagwood sits in his armchair reading a paper, or lies on the couch. At work, there is no couch and I suspect reading the newspaper would not go down well.
Think about you and how you behave at home (kids permitting). At work you dress neatly, you ensure your appearance is professional, hair tidy, good shoes on, nice suit. At home - old tracksuit maybe, hair a mess but you don't care because you feel more relaxed.
Well, we relax mentally too.
Think about yourself or another adult coming home form work. You held it together all day while your boss or your colleagues made stupid decisions and then tried to palm some of the responsibility your way. Or perhaps their mistakes made more work for you. Or perhaps the new whiz kid showed you up. At work you held it together, you gritted your teeth a bit harder maybe but you knew that the price of blowing your stack would be too high and would not achieve anything good.
But as soon as you walk in the door and drop your briefcase, you just want to scream. You rip off your coat, throw your hat onto the hat stand and maybe you do let loose with "AARGH! What a day I've had!" and go pour yourself a stiff drink or find someone to dump on.
Our kids can't do this. They try, but often we are too rigid about letting them vent when they really need to. Or maybe the kid doesn't know how to vent, he just comes home from a stressful day where with superhuman effort he held it together, and just falls apart in a meltdown.
Another factor to consider - medications. As ADHD medications wear off, you can get rebound issues This would complicate the picture for a kid who is having difficulty holding it together. If you leave him off medications over the weekend, he would have more difficulty. But again - if he was on medications, but is in the habit of letting himself melt down more readily at home, he may have been better equipped (during the day on a weekend) to snap himself out of it with enough of an impetus (the video). It may have surprised him that he could do it after all - he's got into baf habits having meltdowns at home.
My suggestion - when he comes home from school each day ask him about his day and encourage him to vent. Get him to explore how he felt in a situation. If he calls people names (such as calling the teacher a jerk or worse) don't scold. At least, not yet. Let him get it off his chest, then maybe discuss. But it's Basket B - if your trying to discuss it with him is pushing him closer to meltdown, pull back. The angle to take is, "How did you feel? Now, how do you think the teacher felt? Why did he do what he did to you? How do you think he was feeling at the time? Can you think of a better way the teacher could have handled it? Let's explore that idea. If it's a good enough idea, maybe we can suggest it to him."
Your son pulled himself together when he had to. That is praiseworthy. Instead, you feel angry. I can understand that, because you are basing this on your assumptions of his behaviour. I think you are correct, your assumptions were wrong. Or to be more precise, they were not quite correct. But you were close, and you risk totally reversing your assumptions into something far more wrong, which is the idea that he is choosing to meltdown, choosing to be manipulative. I don't think this is the case. But perhaps he is more able to control himself tan even he realised. He now has a better idea of his own abilities, as do you.
A point to make to your son, and to also hold in your own mind - no person believes themselves to be all bad (except perhaps for a difficult child with appalling self-esteem). Perhaps a better way of phrasing this - nobody delights in choosing to be an evil person. If you think about the bad people in the world (the ones we think are bad people) - they each of them can justify their own actions to themselves. They had sound reasons for what they did, and believe they were right to do so. Or if they made a bad call and realised it later, they would still say they did not intend to do harm, or that perhaps any harm they did was for a greater good.
In the same way, a child is very unlikely to be spending his day in full control of his abilities and instead of being effective, praised and appreciated, putting his abilities and self-control into "how bad can I be today?"
Your child probably still has less impulse control than other kids, less ability to focus and concentrate. But he does not have zero ability, plus his abilities are improving all the time. A lot of this is maturity; a lot of it is his own efforts. A lot of it is the example and help you and his teachers give him. It all works. But it takes time for it to percolate through.
Home is where the heart is, home is where we feel safe to let our hair down, and home is where we know we are loved and can relax. Let it all hang out. Break wind. Swear. Not have to continue to hold that brick over your head that you've been carrying all day!
Pam, it's OK. He's a normal kid. What you describe is classic and I don't see a manipulative kid in that at all. No more than normal, anyway. And I speak from experience!
Marg