School of hard knocks: "Rocks"

In better years, Rocks might have become a Trainspotting-like cultural phenomenon, not just the small but appreciably shaped word-of-mouth sensation Sarah Gavron's film has emerged as over the past month. It has a similarly effervescent, youthful cast, none of whom you're likely to have encountered before, who bring to the screen new attitudes, slang and rhythms, a freshness you can't teach but which you might want to bottle. With its predominantly non-Caucasian cast and untranslated jokes in Somali, it's a film at ease with the idea of a modern, changing, multi-ethnic Britain, in a way our generally stagnant film industry, for all its hastily applied diversity initiatives, really hasn't been. (Its closest comparison points would be Francophone: Girlhood, Divines, the recent Papicha; there's even a little of Laurent Cantet's Cannes-winning The Class in the mix.) Most importantly of all, it's a piece of sensitive, unforced storytelling, leading us by the hand into the world and life of a teenage Londoner about to have a very great - too great - responsibility placed on her shoulders. Her given name is Shola, though she's commonly known by her contemporaries as Rocks (Bukky Bakray), and - in a crucial year for her education - her depressed mother will flee the family's housing-estate nest, leaving her to try and keep the lights on, while shuttling a younger brother, Emmanuel (D'angelou Osei Kissiedu, the sweetest damn kid you'll see on screen this or any other year), to and from school. Plot may be the one aspect of Rocks that isn't entirely box-fresh: it's the story of a latchkey kid, albeit one given a vivid 21st century remix, with a Ray BLK song over the closing credits.

Gavron's previous projects - most prominently, 2007's Brick Lane and 2015's Suffragette - were assured, but also ever so slightly dutiful: they were the kind of films (literary adaptations, period dramas) which the industry expects from its creatives, where everybody hits their marks, behaves themselves, and goes home with a rosette and something to put on at the Odeon of a Sunday afternoon. Rocks is what happens when a director turns her film over to a cast who have too much energy to ever stay on their marks for long. There are two credited writers (Theresa Ikoko and Claire Wilson) who presumably shaped the narrative line and individual back-and-forths, but most scenes here are rocket-fuelled by the young performers' spontaneity, their capacity for going off-book and coming back with something truer or funnier still: they bust unexpected moves, or give a line a little more mouth and cheek than a Sylvia Young graduate would, resulting in tremendous free-for-alls in the playground and one home-economics class. The film's secret weapon was casting director Lucy Pardee, an Andrea Arnold go-to, who not only found the foursquare, very engaging Bukray, but surrounded her with a coterie of sharp-tongued contemporaries, and then surrounded them with - or, more accurately, had them surround - teachers who really resemble flustered and weary functionaries, doing their best to stem a rising tide of teenage snark. Even the passing hometime chat has the effect of the space dust one of the girls tests on the innocent Emmanuel: it pops and fizzes. You can't get enough of the stuff.

Quieter, but no less present, is Ikoko and Wilson's understanding of what these girls are fighting and hollering about. In recent years, the industry - driven, as ever, by supremely comfortable Hampstead dwellers - has given itself over to the generation of saleable, aspirational images. Rocks doesn't lack for that confidence in filming London that has been such a feature of capital-set films since the 2012 Olympics (as valuable a legacy as any), but it also offers the novelty of seeing a London where people are credibly strapped for cash - where a £29 return fare to Hastings is deemed beyond the financial pale. Rocks starts the film on a rooftop overlooking the Gherkin, Shard and Walkie Talkie - those markers of the ostentatious, sometimes outright dirty wealth currently flowing through the city, placed tantalisingly out of reach - only to spiral downwards after the money her mum left her goes AWOL. The 12A rating serves as reassurance things won't get too grim, and she has a tightly knitted safety net in her pals, as the film's final movement establishes. Here, Rocks gives its characters and cast a deserved break, allowing everyone to feel some sun on their faces, and a glimpse of a better, more leisured life. It's only ever a glimpse, though: Gavron, Ikoko and Wilson sense even this heightened level of girl power isn't enough to truthfully carry their characters any further in this landscape. (Hence their tentative, open-ended conclusion.) Still, if there isn't all that much optimism around in the Britain of 2020, there is more than most in a film such as this: hope for an industry adapting to clear space for these stories; for those creatives out there telling those stories so well; and most of all for youngsters like Rocks and her mates, buffeted and bruised by the moment, yet possessed of a remarkable ability to bounce (and answer) back.

Rocks is now playing in selected cinemas, and available to stream via Netflix.