It is possible to allow patrons to check items back into the school from the OPAC page. 

Navigate to Koha administration > System Preferences and search for SelfCheckInModule.

This will bring up a system preference of the same name.

Set the system preference to Enable the standalone self check-in module and click Save all Circulation preferences.

To navigate to the self checkin page, copy the link in the SelfCheckInModule system preference - /cgi-bin/koha/sci/sci-main.pl.

Go to the OPAC homepage.

Paste the link from the SelfCheckInModule system preference at the end of homepage URL and click enter.

This will open a login page.

Login to the checkin module. Either each patron can login themselves OR a device can be left out with a staff account logged in so anybody can login from the OPAC. This will open a new page where items can be checked in by having the barcode typed or scanned into the box.

Click Submit.

Click Check in.

To end session, click Finish.