At the dawn of a national curriculum for English in Australia, grammar has appeared without any serious interrogation of the terms of its re-entry and against ambiguous evidence about its value for teaching writing. What kinds of knowledge about language do teachers need in rhetorically productive teaching? This article investigates the potential of Halliday's notion of grammatics for understanding students' writing as acts of meaning in context. Drawing on systemic-functional linguistics, I show how teachers can assess writing achievement using "big picture" tools like genre, register and "small picture" tools like Expansion. I apply these tools to two student texts that call for attention to creative uses of language and to excursions and to difficulties with logic and coherence. The paper concludes that a "good enough" grammatics will enable teachers to recognize playful developments in students' texts and also to foster their control of literate discourse.