Underage Magic....

[j. lutz]

No: HPFGUIDX 147467

ereshkigal_doom :
>  
>
>In other words, the ministry can probably only detect magic use if 
>someone is actually watching your location on the underaged-magic 
>crystal ball (or whatever), so you would have to be unlucky or 
>specially targetted to be caught.
>

Also, even if you've been caught, the Ministry clearly exercises some 
latitude over whether or not they will prosecute. In /Prisoner of 
Azkaban/, Fudge smilingly glosses over Harry inflating Aunt Marge. In 
/Order of the Phoenix/, he drags Harry before the whole Wizengamot.

j.lunatic