Most organizations today inherently followed hierarchical access control implemented using a hierarchical key assignment scheme (HKAS). An HKAS enforce reflexive, anti-symmetric, and transitive relations among the nodes (or access classes) in the hierarchy. In real life, the organizations come across rare but practical situations such as anti-symmetric exception, the temporary delegation of access to a user, etc. The traditional HKASs are costly and are not readily implement these exceptions. It motivates to relook at redesigning existing HKASs that efficiently incorporate and revokes the exceptions into the existing hierarchy as and when needed. The current similar work uses asymmetric key cryptosystem to implement the flexible HKAS with exceptions. This work proposes a novel and efficient symmetric key-based flexible HKAS that efficientlyaddresses the flexible hierarchy requirements. To the best of our knowledge, this is the first symmetric key-based flexible HKAS enabling exceptions. The dynamics of the scheme are addressed and compared with the other similar existing schemes. The security of the new HKAS is analyzed formally against a stronger and modern security notion known as key-indistinguishability.