A new option is added to all field that can be added to a database:

"Data can be modified" followed by a list box containing these options :

by everyone (default)
by teachers only
by teachers and the owner only
by teachers and the owner's group only

If a user open a data record for modification and doesn't have the proper permission to edit a field, the field is not displayed and replaced with a message: (modification not allowed)

The attached file contains the files necessary to resolve this as well as entry MDL-5583 (http://tracker.moodle.org/browse/MDL-5583)