Preventing users from sending to multiple addresses.

Google has mechanisms to prevent users from mailing internal distribution groups such as " All Users" but no built-in method to stop them loading up the "To" field with contents of the internal directory.

However there is a way to limit the number of address a student can add to a valid mail item.

In the Apps - GMail - User Settings dialog, highlight the point in the organisation tree below which you want the policy to apply.

Move to, or search for the Compliance section and create a new rule for Content Compliance.

Select all mail heading outside and internal to the organisation.

Select Advanced Content match and Recipients header in the Location section.

Then select Match Type to read Matches regex and set the next two fields to [@] and 6 as shown below.

What you have done is create a rule that will check all outgoing mail from accounts below the selected OU and search the "To" field for the  @ character (which is part of every valid email address) and if it finds more than six instances, process the action.

The action can be anything you like but dropping the mail with a warning seems like an obvious response.