The Ruth wager-concept, in the form of a short fan-fic.
Headmaster: "Miss Thomas, I hope at this point you can see the effectiveness of my methods. The girls have done very well on the test you placed for them, they are well-behaved, studying in their free time, and I think it is perhaps time that you offer me some more leeway."
Ruth: "You have had some luck with these girls, it is true, but you have done well enough under my limitations, and I am unconvinced that you need additional leniency. Your methods seem to be working even with the constraints you are currently under."
Headmaster: "My methods are effective under constraints, but they are far more time-consuming, and the girls have reached a level of discipline I would only classify as 'passable.' If you were to relax some of these prohibitions, I think you would see even more marked improvement in their studies and behavior. I have a proposition for you..."
Ruth: "I admit, you have me curious, what do you have in mind?"
Headmaster: "In the following weeks, you will allow me to demonstrate these methods on you. If, at the..."
Ruth: "You think I will let you spank me?! Surely you have gone mad!"
Headmaster: "... IF, at the end of the period, you do not believe that my methods are the ideal way to modify behavior, I shall relent and never again request any change of the rules, but, if I succeed, you will allow me to administer punishments as I deem best, without putting obstacles in my way."
Ruth: "So, I could get you to stop bugging me if I win this wager... Interesting... I have a few questions. What would constitute winning? Do you propose to improve my behavior? What's to stop you from simply striking me for hours on end."
Headmaster: "You will be able to request a stop to the punishment at any time if I do anything more extreme than the school rules allow, or if you feel that it is too much for you."
Ruth: "So I can have a 'safe' word?"
Headmaster: "In essence, but I propose something with stakes for submitting."
Ruth: "And that would be?"
Headmaster: "If you wish me to stop during a session, you will call out the next digit to the keypad for the girls changing area in the gymnasium."
Ruth: "So you can go spy on the students changing? I hardly think ..."
Headmaster: "So there will be consequences for your giving in. Because if you call off the punishment before completion, that will count as a loss for me."
Ruth: "What?"
Headmaster: "I maintain that I know what I am doing, and I can accurately gauge your threshold for pain, embarrassment, and even pleasure, such that you will be able to stop me, but won't choose to do so, as your subconscious will understand that my actions will benefit you."
Ruth: "So, how can you win this arrangement?"
Headmaster: "If, after a session, you find yourself thinking on it, your mind unwilling to let you forget that it occurred, that would constitute an effective punishment. At that time, you will find me, and tell me the next digit to that combination."
Ruth: "You expect me to come find you and tell you 'you win' after the fact?"
Headmaster: "An effective punishment should last longer than the time it is being performed. Likewise, an effective disciplinarian should not alienate himself from those he must punish. I am trusting you to be honorable, as you are trusting me to not take advantage of the situation."
Ruth: "I can't believe I am agreeing to this..."