Year 9 is said to be the toughest for schoolchildren. The looming GCSE workload hasn’t yet fixed their focus, bodies seem to change by the day, emotions explode and a no man’s land opens up between childhood and adulthood. Sophie Ellerby’s slow-burning debut play, Lit, mostly unfolds over the summer holidays bridging years 9 and 10 in a Nottingham secondary. These are weeks that may be spent reading Harry Potter at home, as the sheltered Ruth does, or partying in a field off the A52 with an older boy, which is where Ruth’s unlikely new friend Bex finds herself.

Bex Bentley (“Like the car – proper classy”) can light up a room with her smile and her filthy wit, which is frequently deployed in raging retorts to her beleaguered foster mum, Sylvia. Bex, like Ellerby’s play, can be both as fizzy and sour as the Tangfastic sweets she demands from Dillon, her new boyfriend, before she’ll take her top off.

Given a sensitive but clear-eyed production by Stef O’Driscoll at the HighTide festival, Lit is about looking for love and the fear of being left – or leaving someone – behind. So the tone is perfectly set in the opening scene, as Bex says goodbye to her newborn daughter even while she is planning an idyllic future for them both. The bulk of the story is framed as a fairytale told by Bex, charting the months that led to her baby’s birth. Each episode is given a short chapter title, held solemnly aloft as a stencilled sign by the actors, casting a shadow on the wall. It’s a curious effect – somehow both gothic and homespun – and jars with the grime and rap soundtrack.

In Minglu Wang’s stark design, a series of metal apparatuses are initially suggestive of hospital screens and later used to represent the caravan where Dillon fatefully introduces Bex to his older brother, who at first seems like a father figure for them both. When the fluorescent tubes of these structures glow in bright colours during the party scenes it only serves to make these characters look even lonelier.

The frequent scene changes feel, at first, awkwardly laborious as the set is wheeled around to a soundtrack dominated by female performers including Cardi B, Little Simz and Leikeli47. But the accumulative effect of these transitions captures the chaotic and disorienting instability of Bex’s life: she is constantly moving between temporary homes. Similarly, Bex goes through a number of outfits, blurring her identities as schoolgirl and young woman, while Ruth remains in her uniform.

There is a vividly volatile performance as Bex from Eve Austin, while Tiger Cohen-Towell shrewdly captures the way that Ruth hides behind a feigned weariness of her peers yet seeks acceptance as much as anyone else. Josh Barrow has the right awkward, self-conscious swagger as Dillon, who sees his first posh date at Pizza Express as the chance to wear his “proper shirt” – the one saved “for weddings and funerals and that”.

While some of the acting is uneven, Ellerby’s script is full of details, light and dark, about teenagers’ initiations to adulthood and realistically charts how their friendships fluctuate and flatline. The predicaments of new mothers in Bex’s situation also feels deeply researched, from the discovery that her unborn daughter has already been assigned a care worker to her fear of picking up the newborn baby in case it makes saying goodbye harder. A blistering debut.

• At Omnibus, London, 17-21 September and at Nottingham Playhouse, 24 September-5 October.