Educated citizens receive a productivity bonus at any job they perform. Educated foresters, for example, will produce more logs per year than uneducated ones, and educated gatherers will find more food in the wilds. Your initial settlers were educated in their homelands; subsequent generations will require a school house to reach maximum output levels.
It is advisable to build a school very early on, in order to keep your citizens educated right from the start - uneducated citizens perform much less efficiently, causing food and resource production to become far less efficient.