I'm helping with a local high school class myself.
First, I gave them a $50 Zizzle and some pointers on upgrading the buttons. This was a nice, easy intro.
Second, I picked up a home edition Fireball for $100. They tinkered with it for a bit, then I picked it up during the summer to rebuild the controls and power supply. You can search for my thread with "fireball arduino." After some mistakes and wrong turns on my part, the system is flipping under microcontroller control, and the kids will pick it up from there(art retheme, switches, controlling the multi-color LEDs, etc).
I also gave them some older parts just to hook up to plywood. They had some fun writing control code for flippers and pop bumpers. I just picked up 24V and 28V parts from ebay to experiment on.
If the kids don't have an existing controller infrastructure already being taught(this school already uses Arduinos), you could just point them at the Open Pinball Project and the Mission Pinball Framework. For hardware, find someone to donate a populated playfield. I personally like the 70-early 80s Williams games. EM or SS doesn't really matter if you are going to build a new controller for it anyway. You'll likely pay less for a non-playing el cheapo project than you would a pile of new parts from Pinball Life or Marco. Someone might even be able to get a tax break for a donation of junk, so check with the school to see how to get that done.

Can they buy a switching power supply from eBay stores? That what my machine uses, along with the OPP power filter board to buffer the solenoid power spikes. Cheaper and lighter than a transformer.

Also, for buttons, an easy to find, cheap option is the Philmore game buttons(30-782). If you live near a Fry's, they usually stock these.
For a really simple demo, they could use an Arduino with a relay shield, like this: https://m.seeedstudio.com/productDetail/2440
A simple program to debounce the switches and fire the relays could get this working quickly. I was even able to use a low amperage 24v wall transformer and three 36v capacitors to fire a demo flipper and pop bumper. Certainly not a production quality set up, but it worked.
Something more robust would be to buy a pair of flipper assemblies from pinball life fully assembled. If you read Vid's flipper rebuild guide, he describes an ideal setup as a replacement for pre-fliptronic flipper assemblies. At least then you aren't debugging a burnt coil or the like on top of everything else.

Ah, I missed a post. That link to pinball life is the same assembly I was referring to. It costs a little extra to trade the coil and EOS switch out for your needs, which is the setup Vid describes: https://pinside.com/pinball/forum/topic/vids-guide-to-upgradingrebuilding-flippers/page/3#post-773606

If they can buy from Amazon, you can get a power supply like this: https://www.amazon.com/gp/aw/d/B01HTH0MUE/ref=mp_s_a_1_4

This board would buffer the power spikes: http://pinballmakers.com/wiki/index.php/OPP#Power_Filter_Board

I can vouch that they are quick and easy to build.

I just bought three of those flipper assemblies for my Flash retheme. Yes, the switches are high current, and the playfield bushing is attached.

You need some capacitors between the solenoids and the PS. It may work sometimes, but you will likely cause the PS to think there is a short on every solenoid firing(this is the short circuit protection you mentioned).
I would suggest building the OPP power filter board. It's not too expensive and nicely solves the problem. Barring that, you might Google for "capacitors parallel solenoid" to see how others have done it. Other than that, make sure you adjust the EOS to avoid burning the coils.

Well, I just built one for the first time, I am slow and admittedly not a great soldering expert. It was a 30 minute job. I spent more time by far on the solenoid driver boards. As described, you wouldn't need the control parts. Just the single PS version which leaves off some parts. http://pinballmakers.com/wiki/index.php/OPP#Power_Filter_Board_3
I keep pushing it because it really is easier than rolling your own. I've done both, and the OPP board will make the lives of the students so much nicer.

I concur. Looks good. When you start mounting parts, make sure to leave room for expansion. The kids might want to add a pop bumper or slingshot in the future.
For now, just buy some posts(or use heavy nails) and add a bunch of rubber bands in triangular patterns to act as passive bumpers.

Nice! I like the launch channel.