The way I've always felt was that I was never challenged, so I didn't expect a challenge when I reached the end of secondary, I had always been capable with the knowledge or instinct I had and I didn't realise it would change. I was also terribly lazy and immature, so I'm not blaming the schooling, it was my own personality and my experience simply backed that up.
I guess I wish I'd been encouraged to reach my potential more, instead of being the problem child who was always finished work well before everyone else and was never really pushed, either within the year group I belonged to by age or within the year group I belonged to academically. I spent a lot of time annoyed that I was doing stuff I felt easy. I was a contrary nightmare I should imagine.
I ended up doing a lot of "helping teachers" and boy did I develop an ego when it came to that, I should have been like every other kid, schooled to my ability level without the special treatment, which I got because I got disruptively chatty when I was finished whatever we'd been set.
I was awful in school. 
I think it is important to have them working to their level, not being held back, but how best to achieve that I couldn't say. For some going up a year might work, for others staying in their peer group and having their work tailored around them might.
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