Problem
Trying to disable share mappings by the dedicated ucr variable does not have any effect on the clients:
ucr set ucsschool/userlogon/create_drive_mappings=false
Solution
There is a bug created for this issue. So far you have to force the resync so the setting will take effect:
ucr set ucsschool/userlogon/create_drive_mappings=false
univention-directory-listener-ctrl resync ucs-school-user-logonscript