Yes, the rain gardens did catch some water.
The Parmenter School had some rain gardens installed as part of the effort to remove phosphorus from the Charles River. By catching the water and filtering it through some natural materials, the intent is to remove the phosphorous before it gets into the tributaries and into the Charles River. Additional rain gardens can be seen along the edge of the parking lot on Wachusett St for the ball fields at Fletcher Field.
Additional photos showing the construction of the rain gardens can be found here