We built a new school, and as the choir had so little in terms of need (versus our band & orchestra program), $4500 for 71 Outdoor Ukuleles was nothing (just one bassoon is $6000). You might not have that budget.
Being in a place that has winter, I think there's a lot to say for buying an instrument without a fretboard. Most schools do not have humidification systems (ours does), nor do many libraries.
For this reason, I'd steer most people away from any wooden fretboard in these conditions. Don't get me wrong...I own a number of ukuleles of my own, and most of them are made of wood. But if you have no way to humidify the instruments, fret creep is a real thing and it impacts beginning players. That includes Dolphins, although I do think they make great starter ukuleles.
I'm a little concerned about the quality control stories I have heard about Waterman. I'm heading to the Texas Music Educator conference next week, and I'll try to remember to bring a fret ruler with me. I'll measure all the Waterman ukuleles that I see.
If you can't afford an order of Outdoor Ukuleles (which sell to schools at a discount rate of about 40% on sets of 15 or more), then I'd also suggest the Flight TUS35. This model is a bit of a hybrid of a Dolphin and a Waterman. It has a wood soundboard and a plastic next and fretboard. I picked one up when the light blue models were selling for $29...I should have bought a few of them, but had not used a Flight before. The only two complaints I have about the model is that the body edge is sharp. Not "cut yourself" sharp, but certainly not a comfort edge. The other is that they are really loud. I have to admit I like the slightly muted aspect of an Outdoor Ukulele (they can get lost in a group) when 50 kids are playing at once, some with all their strength!
As for the colored strings, I love them. They make teaching so much easier. It is so hard to wrap your brain around the concept of an X/Y grid when starting. It is so much easier to say third fret blue string rather than third fret first string (I actually say both). In the video approach that I use with my students, the videos don't all have colors, so the students quickly adapt. Aquila sells them in packs of 20 directly to schools...contact them at their e-mail to order.
The other ukuleles that fit my criteria are either too expensive for a school situation (Fluke, Flea, Bonanza with Corian fretboard) or have mixed reviews (Bugsgear/Eddy Finn Beachcomer, Woodli).
If you decide to go with Kala (e.g. KA-15S), they do have an education site which allows you to buy at a significant discount.
https://education.kalabrand.com/pages/edu-intro
P.S. I've become such a better player over two more years of playing. And I now believe that playing smaller scales can help you to be more nimble on any scale.