I have never heard of a retaining wall holding back a wetland,. I wonder if it was part of the old ditch that used to bring Folsom water. Great photos, by the way.
The Natomas Ditch actually ran well east of the school. Here is a current Google Map overlayed with a 1948 topo map. The green line is the wall as recorded with my GPS.
Here is the topo map only for clarity.