The inquiry centres on claims unfranked stamps were peeled off mail sent to the firm's Darlington offices, and reused or sold them.
Some stamps were reportedly sold on eBay with one rare and unfranked example fetching more than £1500.
A spokesman for the firm, which is headquartered in Glasgow, said: "The Student Loans Company is currently investigating a number of alleged incidents of theft at its Darlington office. We can confirm that some members of staff involved in these incidents have now left the company."
The firm, which employs around 1200 people in Darlington, confirmed that four staff were sacked and two resigned. The company is working with Royal Mail and will "review the information we have available to share with the police".