I think most stuff is already said. I just like to add that I'd not use Sketchup for Design, and give Tinkercad a try. I initiated and supported many school projects over here in Switzerland, and the kids just love Tinkercad. It's playful and just easy to use, and it produces good 3d printable models. Sketchup is known not to produce clean 3d printable models because it was not initially designed with 3d printing in mind. See more information about the projects here: https://en.3drucken.ch/school-projects
We use Ultimaker Original printers in the school projects, because they are pure workhorses compared to Ultimaker 2. They are stable and very reliable. We run 5 Ultimaker Original for 2-3 years now and we didn't have to replace much. In one year with 2 Ultimaker 2 printers, we had to replace many nozzles and ptfe couplers.
1) Spare parts if you go with UM2: Heater block, Nozzles, PTFE Coupler, Heater and Temperature Sensor Cartridges, Print head fans.
2) I'd also stay with PLA, it's easy to print and also save.
3) I'd upgrade the nozzle to a 3dSolex custom heater block, so you can easily exchange nozzles without having to dismount the whole printhead as it is the case when using Ultimaker 2 original heater block/nozzle.
As also already said, I think it's important to say, that there's a need and also a benefit of knowing how to fix a broken 3d printer. If you build it by yourself with some collegue the school will already have 2 people really carring about the printer(s).