Unsentimental and unshowy, here Walker projects the subtlest details as though we’re watching her in close-up. Particularly so in the pivotal moment when this no-nonsense figure, all hands on hips briskness at the start, pauses before ripping up the final exercise book of a batch she has decided to discard, feeling discouraged in her bid to set up a school. In the artless lyricism of sentences constructed by Evans, who has taught himself to read and write, she detects potential that must be salvaged; her dawning realisation is beautifully legible.

Miss Moffat is motivated by a sense that the life of the mind is everything and the genius of Dominic Cooke’s production is that it makes her vision a theatrical reality. Cooke – a star director who keeps triumphing at the National – has gone for an abstract, non-naturalistic approach, creating a haunting memory play out of the old-school script, so that an incarnation of the author himself issues the stage directions and character descriptions.

Gareth David-Lloyd plays Williams as a young man, ill-at-ease in a tuxedo and escaping a society jazz age dance to conjure from the past, and the shadows, coal-blackened, cloth-capped miners, who – lustily singing – act as a melodious Welsh chorus, the mood redolent of Under Milk Wood at times.

Iwan Davies’s brooding, casually clever Evans emerges from the pits of juvenility and stands on the brink of a great escape from all he has known, his sense of self and class in the balance. In the first half, on an empty stage, there’s a rich comedy to the small-town mindsets, typified by rough-and-tumble youths and the snooty, sexist, proudly philistine local squire (Rufus Wright). In the second, we get a proper living-room set by Ultz, and the dramatic stakes are raised, particularly by a sexually bold local lass (Saffron Coomber’s Bessy). A battle of female wills ensues over Evans’s future; without wishing to spoil the twist, Miss Moffat has to go the extra mile of sacrifice to win out, lending her the force and nobility of a tragic heroine.

School-days may or may not be the best days of our lives; what this riveting evening brings home is how crucial they invariably are.

Until June 11. Tickets: 020 3989 5455; nationaltheatre.org.uk