Keep in mind that if there is a higher-level class for the 7th graders who are better at math, the 2-year Algebra option will be depleted of good students. Cohort issues can quickly become more extreme than they were in the earlier, homogenous classes.
I ran into this in high school English, when to get my favorite elective I had to drop to regular English. I had made my own choice to handle the slower pace and was prepared for that, but then found out what a class was like when it had NO honors students left in it. No one answered, volunteered, participated - it was so, so dull. No matter what I did I stood out. I really tried not to dominate the conversation and know all the answers, but sometimes it felt like mentally showing up was all it took to make them look bad to themselves.
Resentment can build in both the students and the instructor, who won't thank the administrator that put your son in the class that was a poor fit. It could even be used as an excuse to undo the acceleration in an extreme case - those social issues will be a bigger issue in a group that doesn't value academics highly.
I'm looking for potential pitfalls here, things to watch out for. If you know the school and this won't be a problem, great! If not...
