Injury before Service

I would. If it's mended by then, then it won't matter to them. If, somehow, you manage to reacquire that injury during training then the staff will be aware of it. If you don't you will probably be jumped for not telling them before.
There's no reason at all why you shouldn't tell them. As long as you can pass the tests, they won't really care.
why do officers say that they are having meetings when we all know that they head straight for the mess and get pished???

it comes back to the same old story.

A young Tom gets sloshed and makes a scene he gets charged and chucked in the slammer.

Then a young officer gets sloshed and makes a scene, it is high spirits and he (ooops got to be pc) or her is let off with it and bought another drink.