I go more by time when talking about long-term mileage for kids since training paces vary so much, but I included approx mileage for each group. Here are the guidelines I use:
7/8 - Build to 30 mins per day. Circuit training instead of a run 1x per week.
Freshman - 30 mins per day (20 mpw)
Sophomore - 40 mins per day (25-30 mpw)
Junior - 50 mins per day (35-40 mpw)
Senior - 60 mins per day (40-50 mpw)
We take Sundays off and do an LR of approximately 20% of the weekly mileage on Saturday if we don't have a race. Some girls progress faster than this and others stay in the 25-35 mpw range for a few years. The biggest factors are 1) physical maturity, 2) training age, 3) training consistency.
For example, I won't have a girl who hasn't hit her growth spurt push the mileage much freshman year, even if she's been training consistently since 7th grade. I've had other girls who train year-round, including the winter, who have hit 50 mpw as sophomores.
Here are a few examples of typical weeks for us:
No race (early pre-comp)
Mon - 20-30 mins of 1 min 5k/1 min steady (beginners can walk the recovery)
Tues - Easy w/ 6-8 x 8 second hill sprints
Wed - Easy
Thurs - 300 meter hills w/ jog to bottom @ 3200ish effort (15-25 mins total)
Fri - Easy
Sat - Long w/ 30 sec pickups
Sun - Off
Thurs Race (Early Comp)
Mon - 3-4 x 2k Tempo w/ 2 min recovery (1ks for girls who run over 5 mins)
Tues - Easy
Wed - Easy (short - 35 min max) + strides
Thurs - Race (higher mileage runners will warmup and cooldown to hit 7-8 miles)
Fri - Easy
Sat - Long
Sun - Off
Long story short, 30 miles per week is reasonable, in my opinion, but a program needs to have progression. I was an assistant coach in a boys' program where the coach had the whole team run in a 10 min range (20-30 mins, for example), every day. He had boys who ran 19 mins freshman year and barely snuck under 18 by senior year--not very good development during that period for anyone, especially boys who are developing like crazy physically.
No high school girl should be doing 90, in my opinion. Even 60 should only come after a few years of adding 5-10 miles per season. You might be able to squeeze a little more out of them, but it's not worth the risk, and I'd rather have them continue improving in college rather than burning everyone out.