Why does ChoChang appear in Hogwarts in HarryPotter&DeathlyHallows eventho' she's 1 yr older than harry?

She was said to be 1 year older than harry and hence must have finished the schooling the previous year itself... ie during Half-blood price. Also there is age discrepancy when scrimgeour meet the trio during harry's birthday... All 3 of 'em came of age but the minister threatens them about arresting them for using magic outside the school-hogwarts.