What age and grade are the members of the Cullen family in?

Well, in Twilight, we are told Edward Cullen is a Junior just like Bella, and so is Alice. The three of them graduate together in Eclipse. Jasper, Emmett, and Rosalie are all one year ahead, owing to the reason they do not reappear at Forks High when the Cullens return to Forks. So, the humans are meant to assume that at the beginning of the series, Edward, Alice and Bella are seventeen, and the rest of the Cullen children are 18. I'm pretty sure that's right.