If you’re only interested in 11th grade, my advice is to look for IB schools. The IB diploma program is an 11th and 12th grade curriculum, so often times those schools will accept more 11th grade students. If you are open to looking at schools outside of the US, you are likely to find many options that admit 11th graders. I know of a few Canadian BS that do this, as well as many in the UK, the EU and Asia.
One option that you may be interested in is UWC. It is fairly prestigious and difficult to get into, but the first year you’re able to apply for is 11th grade.