Well, this is how we used to be able to smoke in the dorms...

We would start off by getting everyone into the "smoking room" (the bathroom in your case). We had duct tape around the door that had lost its "sticky" but still held to the door. This blocked up the cracks around the edge fairly well. Then, we sprayed a little ozium and put a fan in one window blowing in, and a fan blowing out in the other. We smoked blunt after blunt this way.