In Tuition you can add a group of accounts that you pulled up in the Mail Merge tab in reports a Correspondence sent record. To add click on the Sync tab and press on "Mark as Printed" button. It will prompt for a name for the current document sent. When complete it will display a message of how many records have been added.
Even if you are displaying a list of children in the mail merge screen it will only add the document to the account, not the specific student. There currently is no Correspondence table for students.