I can't think of what the movie is called, saw it when I was little and in school... I don't even remember everything about it:

There were some boys that attended an all boy school I believe, and in the middle of the movie, the parents of the boys were eating soup and started all turning into rats.. not sure but I think they had british accents.

The movie was witches. :)
About an academy of witches turning all boys into rats. Classic

Its called the witches.if you go on eBay and type it in you will find it there.theres loads of them

Witches Based from the Roald Dahl book starring Angelica Huston

YA I SAW IT TO!! And started to make a question what movie is. And the first, I remember, the boy play on the tree and there was a witch that came to him, right?

The Witches (1990) - IMDb (http://www.imdb.com/title/tt0100944/)
The Witches (1990) - Amazon (http://www.amazon.com/Witches-Keepcase-Anjelica-Huston/dp/B002GOAH10/ref=pd_sim_d_24)

Luke is staying with his grandmother in a large British hotel. While roaming the hotel, Luke discovers that the Royal Society for the Prevention of Cruelty to Children, which is holding a convention in the hotel, is actually a coalition of witches plotting to eliminate all children. Luke overhears the Grand High Witch explaining their plan to use a magic potion to turn the world's children into mice and they demonstrate it on a boy Luke had the displeasure of meeting earlier named Bruno, but before he can make any attempt to stop them, he is caught eavesdropping and turned into a mouse himself. You follow Luke and Bruno's struggle to survive the dangers of life as a rodent and foil the witches' scheme... by which Luke steals a vial of the potion and drops it into the soup that was specially made for the Royal Society for the Prevention of Cruelty to Children.

http://img.youtube.com/vi/7ugv6tU7sXI/0.jpg
http://i.ytimg.com/vi/GUnXm4vJyPI/0.jpg
http://i.ytimg.com/vi/qoWuW0_OWmM/0.jpg

http://1.bp.blogspot.com/_bDpg9dX2RY0/TBYzO7z9OuI/AAAAAAAABfI/pS_tDNjUDqc/s1600/Luke+as+mouse2.jpghttp://images2.wikia.nocookie.net/__cb20060524010916/muppet/images/0/02/Brunomouse2.jpg