I'm assuming from your post that you have not already applied for this cycle. In that case, you'll need to apply as a new FCPS student in the Fall for Spring placement. This year the Fall deadline was Oct 15 - next year should be similar.
Here's a link that will take you to the deadlines and forms. Reaching out to the AART at the school will be helpful.
Good luck!
https://www.fcps.edu/academics/academic-overview/advanced-academic-programs-aap/advanced-academic-program-aap-forms