I've noticed lately that more and more people are confused about how the whole pupil process works so, in order to help clear-up this annoying problem, I'm going to attempt to break it down for those who don't understand and to be as descriptive as possible along the way.
Let me first say that if you have already created a brute, that brute can NOT become a pupil. It just CAN'T be done so, forget about that right now.
Now, if you do want to become someone's pupil, you must go to their "Challenge Page" which can be done simply by removing the part of their "Cell Status" link that reads "cellule" then pressing "Enter" on your keyboard or you can click someone's link that they are using to request pupils which is basically the same thing.
For instance... http://www.mybrute.com is the link to the main page. If you create your brute from this page, then, you will have NO master and can NOT under any circumstances become someone's pupil.
If you enter the fight this way then, you are a loner who answers to no-one, you are your own master in a sense.
Now, in order to become someone's pupil, you must remove the "www" from the link above and replace that part of the url/web address with the name of the brute whom you want to pupil then, press "Enter" on your keyboard or click the link that they made containing their brute's name that leads to the same page.
You will then be taken to that brute's "Challenge Page", "Initiation Page", "Pupil Page" or whatever you want to call it.
This page will have a message that asks "Are you daring to challenge 'brute's name here'?"
If you want to pupil that brute, you must create a new brute from that page.
Your new brute will have an "initiation fight" with the brute that you wanted to pupil and after that fight is over, your new brute will become a pupil of the brute that you fought against.
Whether you win or lose the fight doesn't really effect the game or your character; even if you were to defeat the master brute, you will still be his pupil and you wont get any points for your victory but, you would still have 6 available fights remaining to help you to level-up.
Being another brute's pupil only benefits you by being associated with that brute but, the brute whom you pupiled will gain exp when you join/pupil them and will continue to gain additional exp as you reach higher levels.
So, basically the master brute gets 1 point every time you do really good but, you still get your 2 points as well so there's nothing to worry about.
I really don't know how to put it any simpler than that. If you still don't get it, you'll just have to figure it out on your own.
Good luck, in choosing a worthy master to pupil.