For a learn to solder class, we wanted to make a project people could solder together and then use, rather than make a useless blinking light like a lot of the learn to solder kits you can find online. Our solution was to create our own Arduino with some branding to support our makerspace.
At first, I began making my own PCB. The hardest part was finding a USB to serial chip in a DIP package, but I eventually found one. The MCP2221A. During my search I came across the MAKIT-THT by Thunkit Electronics. This is what I ended up basing my board off of. You can look at the GitHub repo here or buy a kit from Tindie. Another example of a custom Arduino can be found here The files on GitHub are made in KiCad, and are easily editable by almost anyone. Once converted to gerbers, they can be sent to a service like JLCPCB or SeedStudio for super cheap.
The BOM for one of these boards is less than $10 each from DigiKey. This is a great introduction to electronics and Arduino programming, and allows for a cheap class to teach almost anyone the basics of soldering and microcontroller programming. They also make a great fundraiser/promotional item for a makerspace or something similar.