It is forbidden because:
A) Security reasons. Criminals could come and go as they please. Illicit activities of students would also be facilitated. Further, essentially, Hogwarts would not be a boarding school if students could go home at their convenience.
B) It could go very very wrong. Apparition has been known to cause serious accidents and in extreme cases, death. We wouldn't want a student being sent to Madam Pomfrey every few days...
C) Discipline would be hugely compromised. Imagine boys and girls in each others dorms, pranks, cheating on o.w.l.'s etc etc... Imagine if people like the Weasley twins could apparate within the school... the extent of their mayhem would be doubled.
It could be argued that children should be allowed to disparate to put the method of transportation into practice, but its simply too dangerous and has too many drawbacks. Also, in the case of emergencies at home, etc, dumbledore always lifts the charm and accompanies the students.