To my knowledge, student oaths are something that are specific to your school. I'm pretty sure my Master stole his from the Internet.
Champion - A person who fights or argues for a cause or on behalf of someone else. Freedom - The power of self-determination. Just(ice) - Based on or behaving according to what is morally right and fair.
In simplest terms, you're oathing that if you see something wrong, you're going to fight for what's right, either by directly preventing someone from being abused, or by arguing for better treatment. That you're not going to just stand by and watch something bad happen.
Here's an example I can think of. One of my coworkers was working on a project that had a lot of red tape. He wasn't able to get much done because he was waiting on bureaucrats to approve the next stage of the project. He went on vacation and I took over the project, right as that approval came down the pipe. The customer was telling me how much better I am than my coworker, because he barely got anything done before I took over, and then the project took off. I could've thrown him under the bus and accepted the praise. Instead, I told them how much he had done to get us to where we are, and that I couldn't have done any better at moving us along than he had.
I championed for him because his reputation was unjustly tarnished by the red tape he had to cut through.
The one interesting term in all of this is shall. Shall can mean a few different things, all of which fit in the context of this oath. All of the other words I defined above have multiple definitions, but only one that really fits the oath. Shall can mean:
Declaration of the future
A directive or order
Using definition 1, it states that you will undeniably be a champion for freedom and justice. Definition 2 suggests you would really make a strong effort to be. This allows you wiggle room if you're not perfect, which could be a good or bad thing depending on your opinion. Definition 3 makes it compulsory that you do so, as in that you would face ramifications within your school if you don't.