It's an advantage for those doing the exploiting, but overall it's an economic disadvantage, if your per capita statistic includes the slaves when counting heads.
Yes, educating all people will generate a bigger advantage in total. What I meant was that an individual will obviously make more money for themselves if they have free labor that they can exploit. So a society of N educated people will make less money than one of N people + S enslaved people, but a society of N + S educated people would make even more.
