Nothing in “Lots of kids these days don’t get enough exercise” is a grammar error. That is exactly why it survives proofreading: you are looking for mistakes and there are none. What there is instead is four choices that belong to speech — an empty quantifier, an informal noun, a contraction, and a vague judgement — and the corrected sentence reads as though a different person wrote it.
The leaks cluster predictably. They appear in the final paragraph, written fastest, and in the sentences where the writer felt strongly, because that is when the voice in your head is a speaking voice. Knowing that is most of the fix: the last paragraph is where to look first, and it takes thirty seconds.
What is not a register problem is a short sentence. Simplicity is not informality, and replacing a clear short sentence with a long one at the last minute is the most reliable way to introduce a grammar error into a paragraph that did not have one. The hunt is for spoken choices, not for brevity.
“Do not”, not “don’t”. The cheapest fix in the paper and the most visible failure.
“Carry out” and “point out” are fine. “Sort out” and “put up with” are not.
They hand the argument back to the reader at the moment you should be making it.
The final paragraph. It is written fastest and it is the last thing the examiner reads.
