You can share one or multiple generic attachments with multiple recipients. In this case, all the attachments will be sent to all the recipients. You can see how to do this here: Sending an email
However, it is not possible to mail merge individual documents in bulk to a group of recipients.
For example, if you have individualised letters for each student that you want to attach to an email, you would need to send the email with each attachment one at a time.