You upperclassmen should go for 60 miles a week (excluding your race, which would bring your total to 65 because mile warmup mile cooldown and 5K race). The two underclassmen can stick with 40-50 for now, in order to ensure they can still grow later in the season, and they've got quite a bit of extra talent compared to those 2 juniors, and I'd assume they're better at soccer as well so they've got some better conditioning.
At the high school level in my opinion, its best to be doing a lot of doubles if you can fit them in your schedule. You should do some of your easier runs with the underclassmen, recovery runs with the 2 juniors, at least in terms of pace. Meaning that anyone sub-18 should have easy runs be 7:30 at the fastest (of course you could go down to an average of 7-flat pace), while the two high-18 kids should not be running much faster than 8 min pace for their easy days (perfect for your recovery days).
You could all do your long runs together, but you would make them a little bit progressive. Say you all stick together for the first 4 miles, then you and the underclassmen take off for the next 4 miles, then you solo the remaining 4 at a faster pace. The 18min 5K runners would average 7:45 pace or less for the 12 miles, the underclassmen would be 7:15, and you would be 6:45. If you want to go longer, you could then "cool down" by running back to finish with the underclassmen or even the upperclassmen.
Weightlifting would definitely help you, but of course that's more time taken off your schedule. Do a lot of form-based and support-based exercises, like Romanian Dead Lifts, Step Ups, Single Foot Lunges, etc.
Shoot for at least 4 weeks of this type of base. Stay at this mileage throughout your season until 3 weeks before your last race or first championship level race.
Please note that I'm 100% basing this off of my own training experiences in high school, and what I've learned so far from running. In high school I was a senior at 17:3x, and then trained at 50 miles a week with a senior at about your level (he was doing 60). He ran 9:2x, and I ran 9:5x. for the 3200m.