"Until the second half of the twentieth century, mortarboards were often worn by schoolteachers, and the hat remains an icon of the teaching profession." [source]
So they weren't saying that he just graduated, they were saying that he was learned (the tortoise even says, "he is the wisest of us all").
And owls being symbolic of wisdom is at least as old as as the Greek gods - Athena's symbolism included owls.
"Until the second half of the twentieth century, mortarboards were often worn by schoolteachers, and the hat remains an icon of the teaching profession." [source]
So they weren't saying that he just graduated, they were saying that he was learned (the tortoise even says, "he is the wisest of us all").
And owls being symbolic of wisdom is at least as old as as the Greek gods - Athena's symbolism included owls.
Which is ironic because owls are pretty dumb