The Flint Street Nativity, Playhouse, Liverpool <!-- none onestar twostar threestar fourstar fivestar -->

School play is top of the class

We never meet Mrs Horrocks in Tim Firth's The Flint Street Nativity, but we certainly feel for her. By the end she has passed out in the school playground and who can blame her? The Nativity play she has lovingly produced in her classroom - on a tiny stage alongside the stick-insect project, in front of the wallchart proclaiming "who's been good" - has turned into a complete farce. The Virgin Mary has a fierce competitor in the spiteful angel Gabriel, Wise Man Frankincense is verbally challenged by a lisp, Herod rages like a footballer brutally brought-down mid-match and the Innkeeper behaves like a psychopath in embryo. To crown it all, baby Jesus's head falls off.

Firth began acting as "Boy Three" in a school Christmas production. That career may have been short-lived but the experience has never left him. Some might even say it scarred him for life. Following the success of his television play, The Flint Street Nativity, created in 1999 (before the success of Calendar Girls), and produced with a hand-picked cast of comedy actors, he's developed the idea into a stage version with music. It's clearly striking a chord in Liverpool, where the advance box-office takings are breaking records at the Playhouse.

Unless you're a Jehovah's Witness ("She doesn't do Christmas or assembly," explains one know-all child), perhaps, or have had multicultural, secular entertainment imposed by a politically correct school authority, the chances are that you will either have been in, or had a child taking part in, a Nativity play. No one will fail to recognise the acuteness of Firth's observation - that what a child says or does so often merely apes the back-biting and swear words that he or she has picked up at home, more often than not misinterpreting them, sometimes hilariously, sometimes heartbreakingly.

Firth's canny look at the behind-the-scenes angst goes straight to the heart of the matter. Who's playing the blue-robed Virgin Mary? In this case, it's the precocious, goody-goody daughter (11 gold stars against her name) of the chair of the PTA, astutely portrayed by Gillian Kearney. While the boys seem happy enough to be relegated to playing such bit parts as "the star of Bethlehem" or a cardboard-headed "Ass", the aspiring female actors are already sharpening their talons as well as their acting talents. And when chickenpox threatens to eliminate Mary, bossy Gabriel (Leanne Best) just knows there is a God.

The fluffed entrances, forgotten lines, brazen upstaging and the distractions of excited parent-spotting turn this Nativity performance into a chaotic, mirth-filled charade, directed with gusto by Matthew Lloyd. The mistaking of a scavenged broken tooth for a bit of Polo mint and of a tampon for a fag in Mrs Horrocks' handbag add to the comic misunderstandings of the biblical story. The carols Firth has introduced are belted out to an authentic-sounding school piano accompaniment, though the witty new words interwoven into familiar tunes and phrases are not always easy to pick up.

If, like me, you're normally allergic to adults playing children, this very decent ensemble cast proves it can be achieved with minimum resort to those cringe-making clichés adults fondly pass off as childish habits. While there are more than enough innocent revelations and playground politics at work to create a colourful picture of the parents of these Flint Street seven-year-olds, it's a stroke of genius on Firth's part to bring on the parents after the play has ground to a nightmarish conclusion. The actors return as the mums and dads of the children they've just played, mingling over mulled wine and mince pies. There's no mistaking where these kids have come from or, worse, where they are going. You may never see your child's Nativity play in the same light again.

To 20 January (0151-709 4776)