Old School Blackboard - But How Old???

This is an old school blackboard, wood and slate, nothing more. Its about 3 by 5 feet in size. What I'm trying to figure out is how old it is - anybody have any ideas? How would I even go about guessing?

p.s. am filing this in "Office" because Collectors Weekly doesnt have a School category. and you could theoretically use it in an office. for example, to write over and over 'I will not waste time surfing the internet anymore... 'I will not waste time surfing the internet anymore....'