Other mistake: In the scene where Cameron is sinking in the pool you can clearly see some kind of weight belt or something around his waist, which isn't there when he is out of the pool.
Other mistake: Ferris wants to skip school and who can blame him? In the boring lesson early in the movie, supposedly at the start of the day, look at the student's watches. They all read 6.25. (00:11:10)
Answer: Cameron stares specifically at the child in the painting, because he identifies with it. He feels the lack of affection and attention from his own family (unlike the child in the painting who is enjoying time with her family, the mother is holding her hand).
Shipper