Problem
Users do not have access to classroom shares any more. They might as well have access but no permissions to write a file which another user created.
Solution
The permissions might have been reset for some reasons.
Configure them with the following script to the default values:
cd /home/shg/groups/klassen
for i in $(ls -1) ; do
chmod 0770 $i
chown -R :$i $i/
find $i -type f -exec chmod 0660 {} \;
find $i -type d -exec chmod 0770 {} \;
done