I am trying to create a solution for a client. The client runs a university and wants a system using which they can send the bulk letters that will contain the Marks / Gradi
Signing is an explicit activity by the recipient. There is no way to programmatically sign a document.
It is not-uncommon to programmatically sign documents. If the business process is properly set up, then it is legal too.
Your use case is a perfect example of a valid use case for auto-signing.
The DocuSign Signature Appliance supports auto-signing and that's a common use case for it. For example, the product is often used to automatically sign invoices.
Developer center for the DocuSign Signature Appliance