A good portrait captures the personality...and is interesting...can be revisited many times and perused at length.
There is a reason a JC Penny, Sears or People Place portrait is so cheap. Soul-less and mundane. What kid is soul-less and mundane?
A great portrait is something to be cherished. Something that is forever evocative of the time it was taken. Don't waste you money on cheap portraits...you'll get those from the school anyway.
Invest in a portrait; don't buy a picture.