I worked in the office of First Year Experience my entire time at school - I worked directly with the Dean of Undergraduate education and a lot of people in the admissions office. Basically, you're kid getting in depends on their extracurricular activities.
The expectation of good grades is pretty low if that's all you do. If there's no job, sports, volunteering, etc. and all your kid did was go to school, go home and have 8 hours to do their homework and still get a solid night's sleep, they better have good grades.
But if your kid plays a sport or two, is in band/choir, and does student council, plus church stuff and gets good grades, that really shows something. Outside of Utah, the amount of LDS kids involved in student councils and extracurriculars is disproportionately high to non-LDS kids. In Utah, so many kids apply to BYU that didn't do these things they don't get in.
I have also heard of qualified kids who basically wrote their essays to say "Look at my grades, you're lucky to have me" instead of really trying. They will deny those kids.