Don't boil it, that will damage the skull.
If you want one quickly and cleanly, you can very gently simmer it until the flesh and be pried off. Simmer might be the wrong word... If I see bubbles, it's too hot.
Changing the water frequently can help reduce cooked-in greasiness. Replace dirty water with warm or hot water.
Use a wooden skewer (or something similar) to loosen up the cooked brain. It can usually be rinsed out with any decent water pressure. Something small and softish (like a toothpick) is helpful with removing little bits of flesh.
Soak in peroxide (or diluted peroxide) overnight to loosen up any remaining flesh and whiten it.
If it's greasy, you can soak in ammonia.
Rabbits aren't usually too greasy.
That method won't produce retail-quality skulls, but it works well enough for a kid's collection.