Awesome!
I have some suggestions, and I've led youth trips from San Diego that trailhead out of Agnew Meadows. I won't tinker with your # of days, other than to say: add two and you'll be happier.
I say this because I strongly suggest you do not drive on days you hike, for two reasons -- upon arrival, you and your crew will benefit from acclimation time and, upon departure, your crew will benefit from hot showers and deserve drivers who aren't tired before the 8-hour run home.
So, before I get to what you really asked about, let me suggest you add a day on each end, and that you use the Mammoth RV Park tent camping area for the shoulder days. It's cheap, you can walk to breakfast and the hot showers and pool are "free with camping". Make Saturday/Sunday your drive days, even though everyone else will, too.
Oh, and BUY YOUR PERMIT NOW. They're already available and sell out quickly for this trailhead.
Okay, to your route...
Day 1, I think Olaine Lake is too short for your entry day, and you're not supposed to camp there. If, instead, you continue up the River Trail a few more miles, you'll find great campsites on your side of the river from the outfall of Garnet Lake, just past a trail junction. I can send you some maps if you like. My suggestion is that you use the bus from the lodge to get to the trailhead (plan 1 hour from the time you park until you get off the bus) and then use Olaine for your first stop, a "boots-off" break at the lake, before heading up. This will allow you to do some of the climbing on Day 1, but easily so, and still give plenty of time to explore the river after setting camp. Day One Mileage, 4.5.
Day 2, I think the ascent to Thousand Island will be an eye-opener for the effect of altitude, but it's a beautiful climb and there are great places to use for group-stops that won't block the trail. Add two miles to the point where you reach the end of the lake and you'll be happy-- you can't camp within 1/3 mile of the tip of the lake and the further west you go, the further you get from Bear Central, which is the zone with all the cool campsites and clustered groups of campers starting right at the 1/3 mile mark. Follow the coast west until you reach the fleur-de-lis shaped cove (check it on the map) and you'll have camping and fishing to yourselves. In the morning, you can giggle over the terrified screams of those campers who were savagely mauled by Yogi Bear and...well, you can chuckle when you leave and campers near trail tell you they had a visit in the dark. Either way, you'll be in camp before the afternoon rains and will have time to relax and take in the beauty of the lake and the sunset on Banner and Ritter. Day Two mileage, 6.0 .
Day 3, I will suggest you give the yoots a "zero mile" and Thousand Island's a great place for it. Spectacular sunrise, easy fun fishing, wading or swimming, great day hike routes, etc... But, if you want to hit Garnet that day, consider this a 5 mile day and appreciate that you'll be taking them up over the magic 10,000' foot mark on the topo --pretty cool. Ruby and Emerald lakes are beautiful and Garnet is fantastic, but you're going to have to look carefully for great, legit campsites. Unlike Thousand Island with its vast capacity for camping apart once you move westerly of Bear Central, Garnet has less shore and well-defined limitations where you can camp close to the water. Consider siting up-slope and sending teams for water. Advantage: the overlook sites on the east end give stellar views.
Day 4, if you punch over to Ediza, a beautiful and often crowded place, you'll be popping over another 10k' pass and then you will need to hunt/seek among the very limited camping areas at Ediza. Rosalie Lake will afford more available spaces, if only because Ediza is the more likely stop for more groups.
Day 5, ahhh you get to go past Shadow Lake and descend the spectacular singletrack down past its falls. A great day that takes you down to the intersection right at Olaine Lake, before you finish the day with a mile of ascent back to the trailhead. The low part of the River Trail is even more beautiful as you're heading east than it was going west a few days earlier, and you can start conversations about what the crew want to eat in town. This is a fun day, but don't underestimate the climb out in terms of effect on time. Also, plan for 1-3 hours to get a bus back out to the lodge where you left your truck/cars. Hey, you might get lucky and nail the first one to come by (about every 15 min), but you're the last stop up the hill and the buses are often full. The trick here is to chat up a downhill bus driver that stops and ask that s/he radio other busses about you (they do it all the time), so a bus will arrive with space for you. Then, the dayhikers can grimace at your stench while you grip the overhead bars on the bus and vent your pits over the folks that kept their "good seats", ha ha!! Never fails.
Personally, I think the trip deserves a final night together in town to go over what you accomplished and enjoy walking around town/village after the trip, not to mention the hot showers and night of sleep for your drivers, but if you're heading down right away, plan a major stop at Bishop for fuel (20-30 cents cheaper and it's downhill from Mammoth) and load up on Red Bull for the drivers, who will have cars full of sleeping kids.
You've picked a great place to introduce youth to the Sierra. Keep the miles light and you'll hook a few for sure. Either way, you'll change kids' lives with this, so Good On You.