It is forbidden because:
a. securityA) Security reasons. criminalsCriminals could come and go as they please. illicitIllicit activities of students would also be facilitated. furtherFurther, essentially, hogwartsHogwarts would not be a boarding school if students could go home at their convenience.
b. itB) It could go very very wrong. apparitionApparition has been known to cause serious accidents and in extreme cases, death. weWe wouldn't want a student being sent to madam pomfreyMadam Pomfrey every few days...
c. disciplineC) Discipline would be hugely compromised. imagineImagine boys and girls in each others dorms, pranks, cheating on o.w.l.'s etc etc... imagineImagine if people like the weasleyWeasley twins could apparate within the school... the extent of their mayhem would be doubled.
itIt 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. alsoAlso, in the case of emergencies at home, etc, dumbledore always lifts the charm and accompanies the students.