Hi hhbella. I have quite a lot I could say with regard to the questions you have raised so I am sorry if this is a long post.
We had a similar experience to you. Our DD skipped a grade and then went to an OC for grade 5. We only have experience of that one OC and how well they run is very dependent on the school and the teacher, just like any class. In fact the two OC teachers at DD's school ran very different types of OC so it is well worth meeting the potential teacher and gauging the fit for your child as they will be with that teacher and the students for two years.
Our DD was very miserable in fourth grade but was reluctant to move schools, which she has had to do a lot. We made an agreement with her school that she could return if the OC did not work out well. After the first day at the OC she was a different child, happy and enthusiastic for school again. There were immediate benefits to the OC in that it did move faster and was at a higher level, there were other kids she could relate to and the teacher was understanding. The homework workload increased significantly, of which I was not a fan. Some of the homework was useful and developed academic writing and science skills and some was just hard work.
Unfortunately we had to leave the OC after fifth grade as we changed towns. That was a big disaster. Our DD missed her OC desperately and it was obvious that the level of challenge she needed could not be met in a normal grade 6 class. She was working on algebra at year 9 and 10 level. Her sixth grade teacher allowed her to do that but couldn't support her to do that. We had considered a grade skip to high school but with the move and other issues it became too hard. We regretted that mightily. As you point out, if it is a stand alone primary there is no where to go when they are beyond grade 5 and 6 material unless the school has an arrangement with a local high school.
Like you, we were concerned about the selective school system which struck us as catering more to high achieving students not necessarily highly gifted students. Again it depends a lot on the actual school. We didn't want more work for our DD just the right work. We opted not to go down that path and for us it was definitely the right decision from a "stress" and workload perspective. The advantage of a selective school is they are perhaps well attuned at getting good HSC results and the school we are in does not achieve well at HSC level. Also, there are gifted kids in selective schools as there are no schools really catering to gifted specifically, so friendships may be more easily formed as we found with the OC.
What we needed in a high school was flexibility and awareness. We have fought hard to get that but schools are not set up to deal with students who learn very quickly. Even acceleration becomes tricky as each new grade level only seems to help briefly if it is still moving at a normal pace.
We are now at a point where we have DD doing classes across three different years which is a time tabling nightmare and isn't working well. We are in the process at looking at all options. Homeschooling is hard but not impossible. We didn't do that for grade 6 as we were in a new town and school seemed to be one way for DD to meet new people, although even that didn't work the way we hoped.
On a practical level:
1. There is nothing to be lost doing the test. Put no pressure on it, just do it as an experience and for information. The current school also needs to supply English and math marks as I remember. They are added in with the test score to determine placement. There is no one score to get into an OC. Different schools have different qualifying criteria depending on popularity.
2. Contact the OC school(s) you are considering and meet the teacher and get an idea of the vibe and expectation. What are the OC teachers interests and ideas.
3. Consider whether another grade skip to high school might work better, looking at what is available to you.
