Assuming that the Wizard who made the skeleton invisible did so using the Invisibility Spell (and let's further assume that they used permanency on it so that it would last longer than the 1 hour limit) or otherwise used a spell-like effect (such as a potion) that has the same language...
Anything the target is wearing or carrying is invisible as long as it is on the target’s person. The spell ends for a target that attacks or casts a spell.
The skeleton could make an attack against the wall to allow itself to become visible. Now it's completely undone the Wizard's intent.
Even using Greater Invisibility, which has a shorter duration of only 1 minute and would also need to be made permanent more than likely, the language regarding "Anything the target is wearing or carrying is invisible as long as it is on the target's person." is still there. As such, there are many ways the skeleton could make its presence known:
- throw items
- juggle its sword (or other weapons)
- move an item around
- etc...
Items would be visible while in the air, because they aren't on the skeleton's person during that time. This would be rather conspicuous...
I believe that arguments have also been made that only items on the target's person at the time the spell is cast are invisible. As such, the skeleton can simply pick up an item, and it would appear to float in the air. Skeleton puts on a shirt and some pants, and now it's clothing is visible, even if the bones underneath are not. At that point, I wouldn't even give the players disadvantage anymore...
