To scale all vertices to conform to a certain "plane" in a direction (Y axis), select all the vertices you want to move.
Place the 3D cursor anywhere along the Y plane you want (IE place the cursor's Y position to the position you want your vertices to move to and ignore its Z and X position).
Change the pivot point to
s to enable scale mode, then press
y to restrict scaling in the Y axis. Scale "down" to as close to 0 as possible, or type
0. All the vertices will snap to the cursor's Y position while maintaining their original X and Z coordinates.