ive only taken class pictures but you can ask for an extra copy of the school yearbook if they have any and are willing to let you have one........

i dunno what level you are but I've saved really cool projects that my students did such as the time they wrote comic books...i kept those and laminated them.....

my students also made music videos to a song of their choosing either an eng song or a Korean song if they wrote out the lyrics in eng and I've kept all those.....that would be really cool to do and it doesn't take up physical space in your bags.....you really get to see their personalities and remember other memories with them I think

you may not have time for any of those though

Popsicle stick picture frames and bring an instant camera like a polaroid or if there’s a good printer you can print off hd pics.

You can buy a Korean flag and get them all to sign it or write a comment. This
is pretty standard.

This is a great idea. You may have a flag of your province or township. Ask at the local government office, they should have some and give you one for free.