In their tracksuit bottoms and voice-protecting scarves, the women might well be attending an open-call audition for some big West End musical in search of fresh blood.
A few minutes later they're put through their paces on a metal building-site set in a cheesily choreographed routine involving umbrellas. The bright mood has been salvaged after a piercing electronic shriek. Such an incident gives a hint that what's being knocked into shape is no ordinary musical, as well as highlighting the tensions between old-school "jazz hands" routines and more modern fare.
Such creative tensions are at the heart of Glasgow Girls, one-woman theatrical whirlwind Cora Bissett's follow-up to the Olivier Award winning close-up dissection of sex-trafficking, Roadkill.
Like Roadkill, Glasgow Girls looks to real-life incidents. In this case, Bissett looked to the inspirational tale of the group of school-girl refugees who took on a Scottish Government that had sanctioned dawn raids and the detention and potential deportation of their friends – and won.
Rather than present this as a gritty piece of issue-based drama, and with a slew of producers including Theatre Royal Stratford East and the National Theatre of Scotland behind her, Bissett has opted to transform the story into a large-scale commercial musical. Grime, dub and hip-hop rub up against middle eastern and east European rhythms by way of Scots indie folk, and jazz hands number.
"It's a musical," Bissett gushes, "and we're not going to shy away from that. It's not a play with songs. One of the things that stood out about the real Glasgow girls was that music was embedded in every fibre of the girls' being. You'd see them dancing at home with their families. You see them dancing to hip-hop, but they've also got music from their own cultures."
To capture this, Bissett drafted in three composers to work alongside herself, Brooks and sound designer Fergus O'Hare. All have a diverse musical pedigree. John Kielty is one of three Kielty brothers who wrote The Sundowe, the kitsch, zombie-referencing winner of producer Cameron Mackintosh's TV-friendly search for a new Scottish musical, The Highland Quest.
The Sundowe cast included Bissett, while Brooks provided some of the musical backing. The bulk of the tunes were provided by The Martians, Kielty's band who once included Fame Academy winner turned back-room song-writer, David Sneddon.
MC Soom T is a Glasgow-based Scots-Asian rapper who Bissett first saw playing at an anti-racist benefit. MC Soom T, aka Sumati Bhardwaj, wrote the show's theme song, We Are The Glasgow Girls, with Brooks. The song has just been released as a single to trail the show.
If Kielty's semi-comic songs are the light relief in Glasgow Girls and MC Soom T its furious conscience, then Patricia Panther provides the play's dark heart. Bissett drafted her in to provide a downbeat urban noir for the scenes where law and order swoops. She now also appears in the show.
Finally, Brooks's sister, performer Lorna Brooks is singing coach on the show.
"Glasgow Girls is just on a bigger scale," says Bissett. "I think we've all accidentally discovered a love for musical theatre, but we've been making up the rules as we go along."
Back in the rehearsal room, the Glasgow Girls are back on the floor, and the music they're making is a multi-cultural melting pot of sound and vision.
It's a musical, and it's loud and proud about what it is. Listen closely and it might just be the most radical thing you hear on a stage this year.
Glasgow Girls, Citizens' Theatre, Glasgow, Oct 31-Nov 17; Theatre Royal Stratford East, Feb 8-Mar 2, 2013.