@Charles: I'm not as good at explaining as I would like to be, but hopefully this helps:
How many points of perspective do you use?The "Misty's Big Easy" picture (
http://www.houseofsixten.com/stealth/cirno/illusion/page001.jpg) has three vanishing points: one very near the left of Misty's head (it's still there on the picture), one to the far right of the canvas, and one far above the canvas. The buildings, truck, and sidewalk all respect those vanishing points. And no, I did not extend the canvas and draw guidelines from those vanishing points. I did it all by guessing, which is why the picture has a bunch of errors in the lines projecting from the far right point.
How do you keep them accurate when the vanishing point falls off your canvas?By guessing well. I have a very strong memory of angles and somehow this comes naturally to me - I was able to do this three months into art school. In my experience, other artists who started art school at the same time as me have been able to develop this guessing ability - it usually took them longer than it took me though. And note that my guesses are sometimes wrong.
See attachment 2.In order to perspective properly, you need to "see the unseen".
The first unseen thing is the camera. Where is the eye of the beholder? The lines in your picture appear to be confused as to where it is.
If your camera is a surveillance camera in the opposite corner of the room from the one you've shown, and is near the roof, then the picture would be like:
http://www.houseofsixten.com/stealth/cirno/illusion/classroom-topcorner-camera.jpg - note that the lines where the walls meet the ceiling are nearly horizontal. That's because the camera would be nearly level with those lines.
If your camera is the eye of a person sitting in the second tier of seats, the picture would be like:
http://www.houseofsixten.com/stealth/cirno/illusion/classroom-secondtier-camera.jpgThe second unseen thing is the sides of objects that are blocked from view. The two tiers of tables form a small staircase on either side of the teacher's table. See the dotted lines where the stairsteps are - they will help you position the tables properly.
The third unseen thing is the continuation of shapes. The tables are pieces of a circle (or ellipse). Imagining the rest of the circle will help you draw the part of the table that exists.
That's it for now. Please note, even if perspective stuff is where my "gift" lies, remember that I'm still learning how to do this myself.