That's something my daughter's school has elevated to a priority. They are attempting to formalize the teaching of ASL in the same way that English is taught (and measured). They've begun with the early childhood program, using directed learning and ASL technique pull-outs for students, much like "speech" is done in public schools to refine the ability to deliver a language. I don't know details about where it's going as it was just announced in September and we're only seeing a handful of changes to date, but I really like the intent.
Every student has 3X week "communication" sessions -- one on one sessions with an instructor in which language is assessed and exercised. The focus depends on the individual child: might be expressive or receptive signing, speaking or listening, comprehension, writing, reading. They are watching very closely for gaps, and moving the children into different groupings based on their abilities rather than strictly by age. I know far more about my daughter's specific program than about the individualized programs across the school, but it's an amazing program that several schools have come to observe and plan to incorporate into their schools.
I'm so thankful to my local public school for providing this placement based on my daughter's IEP, and to this amazing bi-bi school that is truly bilingual, developing both ASL and English, and providing individualized resources for children to learn without restriction and without bias using and developing their language(s) and mode(s) in the process.