Which are the best extras to buy if you have to?
Breadboards, lots of LEDs (get red, green and
amber then you can do traffic lights), some resistors (560 ohm) as current limiting for LEDs, PIR and an ultrasonic distance sensor (with a 1K & 2K resistor as a voltage divider). Add a bunch of Dupont wires (get about 80 each of M-M, M-F & F-F) to make the connections between Raspberries and breadboards and breadboards and sensors/LEDs. You should be able to source that stuff within a reasonable budget.
That gets you ready for https://www.raspberrypi.org/learning/ph ... th-python/
which is probably a good starting point once you've gone through the Scratch stage.
You can do some of the physical stuff with Scratch, but I've never looked at that.
Well done, Dougie, for a positive, helpful post (as usual). I second all of those ideas. I can't believe all the negative posts above!
MrsF; welcome to the forums
The items that Dougie mentioned will get you off to a good start ( I would maybe add some buttons, or 'tactile switches' as they are sometimes called) and you'll be surprised how much your students can do with just a basic kit of parts. However, as W. H. Heydt said above, please make sure you get the correct cables for your monitors. Have you checked that they can take an HDMI connector?
The parts mentioned are available from many online vendors. If you have no ethical or patriotic issues with buying from overseas, there are plenty of Far East (China, Hong Kong, etc) sellers on EBay, or one of the larger operations, such as Banggood.com. Shipping is almost always free from these suppliers, and buying in bulk will save you money (500 leds for £5 !).
As for Scratch, I think you're right to start with what you know. If you're happy to stick with buttons and leds for now, then the Scratch that comes as standard with the more recent Raspian images gives you gpio control. See here for documentation: https://www.raspberrypi.org/documentati ... /README.md
If you ignore all of the stuff about add-on boards, you should be able to get your head around it. You may find it easier to look at the 'Examples' folder within Scratch (on the Pi), under the 'File' tab. Or see the 'Resources' page on the website - https://www.raspberrypi.org/learning/ph ... h-scratch/
If you want to do more with Scratch (and it's a lot more powerful than most people realise) on the Pi, try one of the 3rd party add-ons, such as Simon's ScratchGPIO.
http://simplesi.net/scratchgpio/scratch ... rypi-gpio/
Anyway, MrsF, I hope that has helped a bit. Have fun with your lessons, and don't be afraid to come back if you any more questions. There are also other parts of the forums dedicated to teaching and using Pi's in the classroom which, while they might not get so much traffic, the people that do check there regularly understand more about the issues you will have.