If you are working hard, running good quality sessions, the extra easy running may be detrimental. 20 / week is a bit low but truth is that's not
far off the weekly mileage of the first runner to do sub 4' for the mile, and that was on a cinder track, so probably worth 3:55 on a modern track. And no HS runner has gone sub 4 without sub 50 4x400m capability anyway. Speed actually matters in this sport. The other things worth remembering are that if you are out running an interval workout of say, 20x200m w/ 1' rest, or 12x400m w/ 200m walk rest, the VO2 will be elevated for ~30-35' not simply the 11-15' you spend running. A 3' 1000m rep is not the same value as a 3' 600m rep.
On your squad, no doubt there will be some runners destined for XC and 10000m in college -- and ultra running after (they have a hard time breaking 60 for 400m in HS), so they might want to do more mileage and will struggle with the interval work if it isn't properly individualized. Sooner or later though, the speed aspect of their development is inescapable too, even if you are 100% slow twitch you need some speed. The top 10000m and marathon runners in the world have 4x400m capability similar to that of a top level HS miler. Give your coach a chance, judge based on results, not preconceived ideas on what is optimal training.