The tech docs at bellard.org/jslinux/tech.html describe the TEMU config format. Each VM is just a .cfg file - you can see the existing ones linked on the main page. The VFsync integration (vfsync.org) is probably your best bet for getting files into the VM without rebuilding the disk image.

For a classroom with Windows PCs this is close to ideal - zero install, no admin rights, works in any browser. Students get a real gcc toolchain and shell without touching the host OS.