With languages there is an order of acquistition, a natural sequence in which specific skills are more easily mastered. So there isn't really that much leeway in how and when you cover certain points, that is, if you're trying to do it as efficiently and effectively as possible. A curriculum just lays out the master plan, reminding the teacher which skills need to be mastered by when, in order for students to complete the program with the expected level of proficiency. The teacher still gets to decide how to present the material, how to practice it, and how to evaluate student performance.