Well, the obvious reason would be that your school probably has a computer use policy that prohibits thoe types of actions and they also probably carry a stiff penalty, such as loss of access or expultion.

That aside, even if given permission to infect a school computer, which is highly unlikely that any such permission would be given, if it made it into the general population of the school's LAN, it could cause a lot of damage and result in you losing all computer access privelages or expultion from the school.