The environment where teaching is carried out, specifically a school (a metaphor for coalface, after the chalk used on school blackboards).
Etymology
In the education sense, this term is believed to have been coined by Prof. Ted Wragg, for his Times Educational Supplement column. At the time the term was coined, coal mining was still seen as the hardest, dirtiest, most dangerous job there was. The term must have resonated strongly with a put-upon teaching profession as it quickly came into general use.