Working on a Mac, have reset preferences, restarted, etc, but still not working. Everything stays the same.... even my teacher is stunned...
What version of photoshop are you using?
Are you using adjustment layers?
Could you post a screenshot?
What other tools are acting up?
All adjustment layers will affect ALL layers below them, unless you tell the adjustment layer to ONLY affect the layer below it. To do this on Mac, hold down option-key, and click along the bottom of the adjustment layer (to the right of the layer's label). A small arrow will appear to the left, indicating that the adjustment is now linked to and affecting only to the layer below it.
Working on CS6. I tried the adjustment layer, but I was importing the graphics using file\place commands. It wouldn't work no matter what we tried. So, I used the mini bridge and dragged them into the main graphic, and then did the adj layers and it worked. Another misbehaving command is when I'm using the quick select tool, I inverse the selection, and try to delete the background to transparent. When I tried the del command, it comes up as a fill dialog box with foreground, or background colors-no transparent, and it used to work, simply using the del command. As far as I know, I think the other commands are working as they should... I guess time will tell if they are. Do you still need a screenshot the of screen for the del error? I have a windows pc with CS5 at home that I have used for 3 years, so this jump to CS6 on a mac is a little different than what I'm used to...
Thanks for all your help.
Rather than deleting part of your image, just click the mask icon instead ("add vector mask" at the bottom of the layers palette - you may need to invert the mask - Command-I). -- Or, Layer > Layer Mask > Hide (or Reveal) Selection. This is a non-destructive way to "clear" image areas.
Good luck. :+)