This sure sounds like you're saying -- blame the piano. That's not going to work. Unlike other instrumentalists, pianists can't always choose their own instrument. You have to learn to deal with what's in front of you.
I remember one time I was scheduled to play in a recital on a piano I'd never played before. My teacher and I showed up an hour early to try it out. I said something like, this is a terrible piano, I can't play loud enough. Whereupon my teacher sat down and rattled off Rach's C# minor prelude, including the triple or is it quadruple f's. That shut me up.
Maybe you missed the part where I asked the OP whether other, more experienced, pianists have played this piano and whether they can confirm that it is adequate.
It sounds like the OP has already had many hours to try and adjust his piano-playing to this piano, with marginal improvement at best, and he still cannot bring out the melodic line of the Debussy piece he wants to play for his performance, even though he is trying to the stage where he says that is killing him.
I certainly hope that pianists are allowed to have a choice on whether to play a bad piano or not. I play an old, worn-out piano in the rehearsal room the choral group uses, at my church, because there are better ways for my church to spend our money than on purchasing a new piano. But, if I was asked to perform a concert recital on this old, worn out piano, I would most likely decline. There are plenty of other examples of bad pianos out there in the world which sound terrible when played. What if there ISN'T a problem with the OP's technique?
As I see it, there are 2 main issues regarding the OP's situation which need to be clarified:
1) Is there a problem with the OP's technique? If the OP has a teacher, his teacher should be helping him identify and correct this. I still find it rather strange that the OP was happy with how his playing sounded on his own piano at home, and if he has a piano teacher, that the piano teacher presumably was happy with how he was progressing on the teacher's piano during the OP's lessons, but suddenly, the OP is very unhappy with how his piano-playing sounds while he plays on his school's new piano. In my experience, really good pianos actually HELP me sound better, even though my technique is far from perfect.
2) Is there a problem with the piano? Some other more experienced pianist may be able to give an opinion regarding this. If there is a problem with the piano, then there is time to fix it, as it seems like the OP's performance is not something which has to happen in the next 5 minutes, the next hour, or even the next week. Much more importantly, if there IS a problem with this piano, then the OP should NOT be putting in so much effort to adjust his technique in an attempt to make this piano sound good, otherwise he could cause injuries to himself!