Maybe look into something like a Microcarve CNC. It's a little smaller than what you want, but you can probably get 2 of them. I'd suggest 2 because having 20+ kids fighting for time with one machine will be painful. Splitting the classroom up will be way more efficient and allow the kids actually time on the machine.
As for costs, figure another $550 per machine on electronics. $200 on a router and mount. Assuming you get the heavier duty machine (probably wise for a bunch of students) you're talking about roughly $1500 per machine. That leaves $2000 for software, bits, ect ect ect.
For software, you're going to want to look into Mach3 to run the CNC machines. I'd dedicate a PC for each router, so you're looking at 2 copies of Mach 3 ($350). After that, you need design software. The cheap way is to get something like Vectric's Cut 2D and design all of your work in something like Inkscape, Illustrator, Corel Draw, Sketchup ect ect ect. It all depends on what your school has licenses for. Inkscape for example is free and legal to use in a school environment. It compares well to Corel Draw and Illustrator. Draftsite is another free piece of software that is legal to use in schools. It compares well to AutoCAD. I'd personally suggest sticking with 2D and 2.5D carving at the moment and work your way up to 3D stuff. 3D stuff is very time consuming and will be hard to run in a 30-40 minute period. Many 3D carvings take hours and hours if not days.
Microcarve MV1 = $599+shipping
Nema 23 Electronics Kit = $520+shipping (cncrouterparts)
Bosch Colt Router = $100 (Amazon.com)
Bosch Colt Mount = $55 (K2cnc)
Switches & Misc = $50
Mach3 = $175
Vectric Cut2D = $150
Inkscape = Free
Draftsite = Free
Total = ~$1800 (including shipping). $3600 if you buy two machines which leaves you a healthy budget for materials and bits.