Hammering it in from 500-700 meters is not usually a tactic that most high school kids can handle. You really want to run even pace most of way skirting the anaerobic threshold.
If I had a kid trying to run under 5:00 for a mile then I’d really want them to be around 2:23-25 for the first 800 because it’s not too likely that they will run negative or even splits because again, that is a fairly advanced thing to be doing until you have a really good sense of pace.