Hello!
The GREAT HALL/ENTRANCE HALL is open to the students and professors already at any moment, though the official start of term is not until October 29, real time. This means, until then, the rest of Hogwarts is NOT open. Please make sure to post in the Exam Evaluation and Sorting Hat. This is so you have a year and house.
There is NO posting requirement per term, just a small task per year for each student to do that is only one task. Once it is completed, at the end of term (which is undecided) you will graduate based on if you finished the task. You will be given your task at the start of term.
Until then,
Godric