"Your essay is too advanced in vocabulary and sentence structure: you really must try to dumb down."
I believe you really, really don't get how the dumbing down happens in schools. Teachers don't give kids instructions to dumb down. Teachers do it themselves. For example, in English lang/lit spelling mistakes are often overlooked because a) worrying about spelling hampens the flow! b) not everybody can spell brilliantly so we have to "even the playing field".
Remember what Labour did to spelling in GCSEs? No minus marks for misspellings. Not even in English papers! So what incentives for teachers to get pupils to spell correctly? Some may do, but overall it's not a priority.
Teachers routinely tolerate bad grammar as well. "You was", "them books" etc., are often overlooked because, well, the kids are used to those expressions among their family / friends.
That's just in English. In maths it's a whole lot worse! Teachers themselves are often unable to appreciate more efficient ways of dealing with a problem. They know one way, they stick to it. For example, I taught my DS the technique often associated with Trachtenberg for quick multiplication. Take multiplying by 11.
243524 x 11
Traditionally:
243524
x11
_
0243524
243524
_
Add the above
However, if you look carefully at it, it's simply a matter of adding adjacent numbers. So you can skip all intermediate steps and put down the answer down as (0+2), (2+4), (4+3).... (4+0).
There are similar rules for multiplying by 6, 12 etc. Once mastered this is blindingly fast even with larger numbers where carrying is sometimes involved. My DS is capable of recognising where this technique is best used and is very proficient at using it. He can see the pattern in multiplying by 101 (add the neighbour's neighbour). He can square 46714 in less time that you can type it up on a calculator. But teachers don't know these methods and even if they do they have to stick to one (inefficient) method to ensure the whole class can understand. Dumb! But essential. We always insisted that when DS is in school he should use the method the school prefers ...and he always complied. Till one day I saw him doing his homework in the study and using his fingers to add 15 and 4. Why? Because that's how they do it in school!
The carefully cultivated anti-competition environment in schools doesn't help. Some children need competition or they won't give their best. But schools aren't worried about you giving your best (if you're in the top few percent).
I could go on. Dumbing down happens in numerous and subtle ways. Anything that doesn't expect and demand a child's 100% best effort is dumbing down. Anything that praises a child's half-hearted effort because it happens to be above the average expectation for that age is.... dumbing down.