FreeCol:Colopedia.concepts.education.description/en-gb

The most efficient way to educate your colonists is to build a schoolhouse and assign an expert unit as a teacher. The teacher will then turn a free colonist into an expert of the same type. Teachers are also able to upgrade petty criminals to indentured servants, and indentured servants to free colonists. Some skills can also be learnt from the natives, by sending a free colonist or indentured servant to live in a native settlement for a turn. And some skills can also be learnt by experience, by putting a free colonist to work a colony tile. Free colonists working in the fields and forests surrounding your colonies accumulate up to 200 points of experience, which amounts to a four percent chance of being upgraded.