French expat here.
Bilingual kids often attend weekend classes in their native or acquired language outside of MCPS and then take the corresponding AP exam, which is recorded on their MCPS transcript. The year they wish to take their AP exam, they register at their MCPS school, and show up for the exam there.
My kids have attended or are attending Les Classes du Samedi
https://myfrenchclasses.org
and in 10th grade, my oldest took a semester of prep for the AP French, and a semester of prep for the DELF B2. The DELF B2 registration and exam was arranged by Les Classes du Samedi. The exam took place at Rochambeau, the French private school.
Thanks to taking an AP in a foreign language, kids can receive the seal of biliteracy on their high school MCPS diploma.
To fulfill the language requirements of MCPS, my oldest picked Latin, which he studied to AP level. And my current high schooler picked Spanish, which she will also take to AP level.
If one of your children are of intermediate level in French, and you want them to learn French within MCPS, they can take a placement test at their school - contact the language coordinator to arrange it. However issues arise when the kid has too high a level for the middle school they are going into. It used to be that middle schools would bus students to the high school for first period language or math (so Level 4 of any language or Algebra 2 in math), but MCPS has curtailed that program signficantly and a lot of middle schools don't do that now. This means that your kids might be stuck with low level classes before they get to high school. But if they are all on track for 8th grade DELF B2, then they might be bored!
This is why bilingual kids often go outside of the system.