My kid did this when she was in 6th grade. The rehearsals were a little far from our house, but there weren't that many of them, and it was fine. Schoolwork was definitely not affected! Please reassure your kid as to this point.
For context, she was concertmaster, having started playing when she was very young and been for several years in MCYO (Maryland Classic Youth Orchestras). I think it's a great idea for your child to try All County, and then think about joining a youth orchestra!