Skeletons drop experience orbs when killed by tamed wolves.

The skeletons dropping exp orbs is intended. It's kind of hard to explain. A tamed wolf is treated as a weapon used by the player, and any player involved attack on a mob will grant exp orbs upon the death of the mob.
The feature of tamed wolves automatically attacking skeletons and skeletons flee rather than attack is intended. The problem here is that you are abusing the attacking through a closed door glitch.
exp farms are unintended side effects of certain glitches or certain features added to the game, and many of them have been removed, just as this one should be removed.

Intended.