Instructions
Enter the first stall on the left and repair the pipes to the toilet.
Head over to the far left side of the room and hit the wizard portrait on the wall to knock LEGO pieces to the ground. Assemble these pieces to clear the pipe entrance.
Swap to Ron and send Scabbers up the pipe. Have Scabbers crawl through the pipe to hit the switch for the first toilet, flushing out LEGO pieces for the snake statue.
Cast Lumos to untangle the next stall to the right. Enter the stall and repair the pipe fixture to clear the pipe pathway.
Cast Lumos on the third stall to remove the vines and repair the toilet pipes similar to Step 1.
Return to Scabbers and press the last switch in the pipes, making a frog jump out of the toilet. Chase the frog to the closet on the left.
Cast Wingardium Leviosa on the closet to force the frog and other LEGO pieces out. Assemble the LEGO pieces to create a plunger. Ride the plunger to the toilet in the third stall to find the remaining sink pieces.
Assemble the sink pieces and swap to Harry. Stand on the circle of light to speak parseltongue and open the secret passage way. Jump down into the chamber to continue past the bathroom.