What happens when I destroy a building

The reason you can't trade your items with the merchant is because they have a trade value of 0. Certain merchants won't accept certain kinds of good for trade, and so the value of those goods will be set to 0 when trading with them. An example of this is herbs, which a lot of merchants won't accept for trade, but food merchants will.

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.