The bus is typically the easy part. If there are staggered start times for elementary, MS and HS, then the elementary kid just takes the MS bus and math is 1st period. Then they get shuttled back to their school. If there aren't a whole lot of kids in Algebra, then they could just use a school van. Heck, the district maintenance staff could drive them back. Won't help your kiddo now, but for future kids in the same situation, maybe worth a mention.
In our public SD they absolutely will not transfer subject-accelerated students between schools. We were told that that is a parent responsibility we should be aware of should DS be accelerated. Also, the elementary schedule starts just 5 minutes later than the middle school schedule which starts just 5 minutes later than the high school schedule and the class times do not line up at all between schools.
