Sending, receiving, and organizing email in Outlook.com
Hello,
Microsoft does not directly manage organization users. Instead, each organization's IT administrator is responsible for managing its users according to its internal protocols.
Whether you should still have this account after graduation is the organization's internal decision. If that's their policy that you can keep your account active, and you cannot get in, contact your school's IT department. They can reset your security, including password for you via Admin Center, a portal Microsoft provides for organization admin users to manage their organization account, which includes their internal users, permissions, licenses, content, policies, etc. Microsoft doesn't have permission to directly manage organization users bypassing their admin users.