Private schools recognize that students receive editorial guidance on their admission application essays—as would anyone with a significant writing project. From this perspective, it is concerning if a student submits an essay with spelling and grammatical errors. So, yes, you can edit your student’s essay, but it’s important not to cross the line from editing to writing.
The essay as a whole must remain the student’s work. Private school admission committees review hundreds of essays each year. They have a strong understanding of what a student at each grade level is capable of producing, so it’s likely that over-coaching will do more harm than good.
How much editing is too much? Spelling, punctuation, and grammatical editing are perfectly acceptable, as is offering overarching feedback: “Your intro is strong, but you could tie the theme together better in the conclusion.” It’s also good to provide thought-provoking questions: “Have you thought about including [fill in the blank]?”
As much as we want to help our children succeed, just remember that it’s good to make suggestions, but it’s up to them to write the essay in their own voice.