Despite being upgraded to 2.3 stable, the School demo site is still suffering from the issue here http://tracker.moodle.org/browse/MDL-33877
There is an additional problem with previously marked and upgraded assignments not displaying grades or feedback for students as mentioned by me here http://moodle.org/mod/forum/discuss.php?d=204996
Not sure if the two are connected?
Steps to reproduce issue in
*Log in to school demo as "teacher/pw = moodle"
*Go to the Biome study course and attempt to edit the settings of the travel brochure assignment here http://school.demo.moodle.net/mod/assign/view.php?id=1363
*You will get "Found more than one record" etc etc"