If you have concerns regarding accelerated GT education creating gaps in your child's knowledge (this is what I understood from your OP) - then, ask them how they implement gifted education - ask if they compact a regular curriculum to do more than a year's worth in a year, if they skip certain parts of a curriculum to accelerate, if they pre-test before skipping and compacting, if they provide "depth" rather than speed in progression, if they do "enrichment" in addition to normal instruction or if they use specific methods catering to gifted kids (project based learning is one example). The best way to assess whether your child has gaps in knowledge or not is for them to take a standardized test (ERB or ITBS is what local private schools use). The standardized tests are a battery of tests that give a break down of scores in all the areas of learning that you can review to check for weaknesses. Standardized testing is done by most private schools and they use the results as part of their marketing strategy - so, they will share the school's performance by grade with you if you would like to know where the students of the school stand nationally.
As for switching over to traditional high school, it is going to be difficult if your child is ahead by more than 2 years in any subject. locally, I have heard of stories of kids switching over from private schools to PS for HS and having to repeat a large chunk of chemistry and biology because they seem to have covered most of the 9th grade material in 7th and 8th. But it really depends on the school district and its policies. Some districts allow kids to go over to AP level courses if they have transcripts proving that they have completed qualifying course work (not happening in my area, but, it might be possible elsewhere).
