What grade would i be in America?

One big difference in America is that our school years are split between two calendar years (rather than just going from January to December), due to the seasons being different than in the eastern hemisphere. Usually it's between late August or early September to late May or early June. So next school year, for example, would run from August/September 2016 to May/June 2017. Also, we don't get as many breaks throughout the school year as you probably do in Australia, which is why we have such a long summer break in between school years.

Anyway, with a September birthday, you'd usually be the oldest in your grade, but it would depend on the cut off date for starting school in your state (since there isn't a national cut off, each state can set their own). For example, 15 is the general age for starting 10th grade, but most states say you have to turn 15 before September 1st (or in some states it's even earlier, like August 1st). So that means since you turn 15 on September 7, 2016, you just miss the cut off and would go into 9th grade. But then there are some places like New York where the cut off actually isn't until December 31. So there, you would make it into 10th grade. But then again, since you're older and at the high school level, I would think that they might place you in a grade based on what you've completed so far in your home country rather than your birthday.

School is only half way over in the US. If you came now you would finish the 8th grade if you came after June then when school started again you would be in the 9th grade.

You would be in 9th or 10th grade depending on the school and your birthday.

Seeing as you are 14, you would have started 9th grade around August or September of this year (2015).

Yes, unless school starts on or after the 7th.

9th grade.