Thanks for your ideas...
> I would add the Raspberry Pi 7 inch touch screen.
This is a 800 x 480 pixels resolution screen. The RPi is meant to be capable of 1080 vertical lines per frame. With a 16:9 aspect ratio then this is a monitor of 1920x1080 pixels. This seems to be a common pixel ratio with HDTV's. Thus I suspect that due to mass production rates to fill the digital TV market, the price of these monitors should be dropping the fastest.
So I would like to see a RPi with a 1920x1080 pixel monitor. I see that these monitors are commonly available in 21.5 and 24 inch. It looks like the 21.5 inch seems to be less popular and prices have dropped below $200. However getting one of these monitors with touchscreen seems to be a bit expensive at the moment.
I see there is a company called gechic. They make some touch screen 1920x1080 monitors at 15.6inch. Models include the On-Lap2501H and On-Lap 1502. If you look at...
...then there is a photo of a RPi connected to one of these gechic touch-screens.
So, maybe it won't be too long before there will be good-sized 1080p touch-screen monitors suitable for RPi at a low price.
> Use power banks (super cheap from China). No power in classroom is needed.
While it would be nice to be rid of all cables, I think that power banks may be a bit messy to manage. You'd need to put 28 of them onto chargers each night, then hope they lasted the 6+ hours of a school day. If they only produce +5V for the RPi, then you need to also get +5V monitors. Plus their life-span of delivering to specification may not be very long. So I figure having an AC to DC regulator for each row would be more simple and then having DC cabling distributing the power down the row.