Interesting indeed that so many high IQs co-exist with some SpLDs. In my case it's ADHD, not a known thing when I was at school. We had IQ tests when I was about 10, but when the outcomes were being given to pupils to show our parents, the teacher stopped at my desk and said "This can't be right. I'll get it checked". Heard nothing further.
School performance was up and down, a model pupil in the interesting classes, a disruptive delinquent in the boring ones. Didn't get great qualificiations. Like a PP I joined Mensa to prove (to myself mostly) that I wasn't an idiot. Decided to have a crack at college as a mature student just six years ago, found it too hard, but on the advice of an acquaintance I spoke to my GP, got assessed and diagnosed for ADHD and got all the support I needed to finish college, then went on to do a degree. I nearly quit so many times but I made it.
I can understand schools not picking up on these issues one way or another. When I was good I was very, very good, but not enough to be outstanding. When I was bad I was below average - how would anyone have given me a second thought? Having a high IQ with a LD, especially when many of them are still not fully understood, will always mean that each issue somewhat tempers the other, making them both much harder to spot.
My study support worker, on hearing of my very late ADHD diagnosis (50+) told me that after the diagnosis I needed time to grieve for the life I could have had, which I thought was a tad dramatic. I don't feel let down by anyone, or by the system, my life may or may not have been different, but that doesn't mean I regret the life I've had.
I eventually took the Mensa thing off my CV because no one was impressed and it started to feel a but wanky. But I retain my membership because I'm so unused to achieving anything that I keep thinking my degree was a fluke, and one day the uni. will realise their mistake and ask for it back! Whereas Mensa will only throw me out if i stop paying.