Who is the headmaster of Hogwarts for most of the series?

The story behind the answer

Albus Dumbledore is headmaster of Hogwarts for the majority of the series (Books 1–6), guiding Harry and the school through Voldemort’s return. Though there are brief interruptions—Umbridge’s takeover in Book 5 and Severus Snape’s tenure in Book 7—Dumbledore remains