Julian the Apostate was forced to drink molten gold after being captured by the Turkish sultan... an ironic end that satisfied his many Christian enemies across the border. He was then skinned and his skin was tanned, stuffed, and hung on a wall outside his palace.
Seriously overkill for a guy who just wanted to reopen Delphi and sacrifice a hecatomb.
Leidenfrost effect. Since the boiling lead is 1740C, the saliva in his mouth boils instantly keeping a layer of steam between the lead and his mouth so that little heat transfers. When it has cooled enough to be solid but not cooled enough for the Leidenfrost effect to fail, he spits it out. So long as everything the lead touches is wet and as long as he spits it out before too long, he won't get hurt. It's still super impressive.
I know it works, but how does that work! Even steam is hot enough to cook you, and this isn't like briefly sticking your wet hand into a vat of the stuff.
The difference between these guys and your old science teacher is their sheer pain tolerance, though my 11th grade science teacher was no slouch in that department either.