I used a scratch teacher account. I pregenerated a list of usernames in Excel and uploaded them as a CSV file to the scratch site. I then mail-merged them into a word template and printed the result so each student could take home a slip with their details on. I also recorded who had which username on a sheet, as inevitably a couple had forgotten by the next session. I normally login to my teacher account to demo the project anyway, so it's only a couple of clicks to reset any forgotten passwords during the session, which saves a lot of hassle.
Here's my word lists I used for usernames, excel randomly chose one word from each set so you ended up with some fun combinations to choose from.
Fire, Dyna, Doctor, Wonder, Super, Astro, Space, Night, Deadly, Purple, Red, Flash, Green, Candy, Cookie, Fluffy, Rage
Storm, Blast, Wookie, Muffin, Rocket, Thunder, Falcon, Cat, Dog, Skull, Hand, Boots, Laser, Crash, Crush, Doom, Zap
Example resulting usernames: