The theme is enticing. A massive Sapphic crush at an elite girls’ boarding school, with lashings of sex and agonised longing. The debut from K Patrick, one of this year’s Granta best of young British novelists, Mrs S promises classic hothouse drama then segues into an experiment with form that frequently converts expectation to bemusement.

A nameless 22-year-old teacher from Australia arrives at a nameless posh school where the nameless girls worship a nameless dead author. The narrator, a lesbian woman, has taken on the position of matron. “Miss”, as she is known, befriends the housemistress, who is also queer, and pretty quickly clocks the headmaster’s wife. This Mrs S is the only character who is halfway named and is an exemplar of heteronormative femininity in her skirts and scarves. Let the throbbing begin.

Our protagonist observes her charges as they indulge in some well-captured adolescent neuroses and struggles with her own role in a new, restrictive environment. Though there are hints that suggest we’re in the late 1980s, the era is unspecified, obviating the need for arguments on identity politics and allowing the themes of gender, sexuality and desire more subtle exploration. The narrator is butch, or in current parlance, possibly transmasc; she wears a chest binder and is painfully aware of her appearance, her accent and her general outsider status. As the summer warms up, so does her fixation with the older and distant Mrs S.

There are set pieces – violence at a school dance, a Lorca play, gardening, more gardening – that form a background to the progression of the relationship, but the 300-odd pages of this novel are a slow burn. With its suppressed yearning, erotic tension and search for the “self”, the book is essentially a lengthy prose poem that will delight some readers and alienate others.

Mrs S is inventive and original in many ways, and very much of its time in others: reflective, solipsistic, essentially plotless. But Patrick at their best is a powerful prose writer, with dense, intense yet pared-back descriptions: “The whole uniform is done in an awful blue. A cheapened summer sky.” School dinners are “bacon with shimmering, rubberized fat. Damp toast. Soft apples. Pears with snakeskin.” There are valid comparisons with Garth Greenwell to be made, and a Woolfian stream of consciousness is definitely at play.

But the stylistic choices present some serious problems of pace. Lisa Halliday’s description of a novel her character is reading in the opening page of Asymmetry captures the style of Mrs S exactly: “It was made up almost exclusively of long paragraphs, and no quotation marks whatsoever.”

While a lack of inverted commas is commonplace, the solid chunks of prose are devoid of either line breaks or speech tags, forcing the reader to pause and go back to try to work out who is speaking, often in vain. To add to this, the entire novel is written in short sentences with very little variation in rhythm; the stop-start, staccato style can become relentless. A typical example: “I’ve broken a rule. Only Mrs S remains. I can’t see her face. Her hat, the stupid, wonderful hat. Is she proud of me? She is unbearably still.”

The novel’s strength lies in the ratcheting up of longing, culminating in passionate sex as the heatwave summer continues: “Without waiting for me she removes her white shirt. Each button a piece of my own spine, undone … On the knuckle under my nose, barely a trace of her but enough. A smell that doesn’t belong to me. Different chemistry, private.” The butch experience and the protagonist’s vulnerability when confronted with heterosexuality are movingly conveyed, as is the portrait of enchantment: “When she is not around, I invent her. When she is around, I invent her.”

Atmospheric and daring and at times beautifully written, Mrs S would be more powerful as a novella in which the avoidance of conventional fictional devices in a shorter form would elevate its originality above its own challenges.

• Mrs S by K Patrick is published by 4th Estate (£16.99). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.