When should I set up a school?

A few facts to consider:

• Students graduate at age 16 if they live next to the school. If they have to walk (back and forth) to the school, that walking time does not count toward education progress. This is why players are reporting 20 year old students.

• Education gives the worker a ~50% productivity increase. However, education does not make a worker walk faster. This means that hunters, gatherers, foresters, laborers, vendors and tradesmen do not benefit much from education. Fishermen, miners, quarrymen, firewood cutter, blacksmith and tailor all benefit from education.

• Having students locked up in school delays the creation of couples. This could tip a city that's on the edge into stagnation. If you have fewer than 10 children, you should probably be focused more on making more children, rather than education.

• Education also reduces random accident death, so you'll want it eventually.

So, putting it all together, I'd build a school when all of these criteria are met:

• There are more than 10 children.
• There are more than 8 workers in non-walking jobs.
• There are no more pressing needs (food/firewood).
• There are at least 4 spare laborers (+1 to be a teacher) to handle the gap created by the first wave of students.

Building a school and training students to completion causes the age of maturity in your town to move from 10 (months) to around 20. That means that for 2.5 years you will have people die of old age/other causes and not be replaced. If you can afford that, based on your stockpiles and productivity, it is advantageous to open a school.

There's almost no time too soon to build a school. Educated people are more efficient and also have less workplace related accidents. That's a force multiplier that's hard to pass up. As soon as your people aren't going to freeze or starve, put up the school.