If I were involved with teaching, I would be all over Arduino. The Arduino Nano is $10 at Amazon and the very popular Arduino Uno is $35. There are Starter Kits (of parts) all over Amazon and some come with really good documentation and tutorials. VERY good tutorials! Plus, there are projects all over the Internet and hundreds of videos.
Agreed. I really can't see any point to use PIC at this point, unless you're someone who's been using it for decades.
The ELEGOO Uno clone is $14 by itself. I don't notice any quality difference between them and official Unos. I think it's only $1 more for their Mega2560!
ELEGOO also have a pack of three Arduino Nano clones for $16, including headers you can solder on if you want.
Arduino Nano of course has the usual ATMega328 with 2 KB RAM and 32 KB flash, same as the Uno.
For projects that don't need many pins (5 IOs if you don't want to risk easy reprogramming by repurposing the RESET pin), it's also fun to use bare DIP8 ATtiny85 chips, which don't require any external components at all, cost about $1, and can be easily programmed using an Uno.
For code that uses mostly 8 bit variables with a few 16 bit thrown in, the AVR instruction set is just about as friendly to both the compiler and the assembly language programmer as ARM, especially the cut-down ISA in the Cortex M0(+).
AVRs are a *lot* easier to program bare metal without using library code to initialise the hardware.
The $4 Raspberry Pi Pico is also a great new choice, with 133 MHz, 264 KB RAM, 2 MB flash, dual Cortex M0+ cores, and very comprehensive and understandable documentation.
The $5 Sipeed Longan Nano is a dark horse contender. You get 108 MHz RISC-V CPU, 32 KB RAM, 128 KB Flash. As a bonus, the $5 price includes a 160x80 RGB LCD display. The documentation isn't as good as Arduino and Pi Pico, but it's not bad. You could certainly build courses around them. They're supported by the nice PlatformIO IDE.
It's really quite amazing that you've got good AVR, ARM, and RISC-V options for around $5 a board, all with easy power and programming via a USB connector.
At those prices, you don't care if students break a few of them.
These $5 boards look not quite as convenient as an Uno to get started with, but add the header pins and plug them into a cheap breadboard (which you quickly need with the Uno anyway) and it's basically the same.