No kidding. There is this little button on the monitor that glows, when you push it the image goes away. What was she thinking?
That aside, I think that the school should take some responsibility here at least. They didn't provide the proper protections to prevent this kind of thing from happening. If you can't afford the bill (which I doubt), google has a free pop-up blocker. Come on! It may not be perfect, but at least try.
Not to mention, it said that the regular teacher was out of the room. What was he/she doing, and where is his/her responsibility in all of this? I am a full believer in the concept of taking responsibility for one's actions, but this is a little excessive.