Exceptional groups of order $p^6$ for primes $p\ge 5$

The minimal faithful permutation degree $\mu (G)$ of a finite group $G$ is the least integer $n$ such that $G$ is isomorphic to a subgroup of the symmetric group $S_n$. If $G$ has a normal subgroup $N$ such that $\mu (G\mathrm{/}N)>\mu (G)$, then $G$ is exceptional. We prove that the proportion of exceptional groups of order $p^6$ for primes $p\ge 5$ is asymptotically 0. We identify $(11p+107)\mathrm{/}2$ such groups and conjecture that there are no others.