As a school principal one of your duties likely is to handle student discipline. Nothing can derail an investigation into an incident like misinformation. Many students that see something happen refuse to cooperate because they either are worried about possible repercussions to themselves or they don't want to "snitch" on their friends. Having a misinformation policy in place will not get everyone to cooperate, but it will encourage others to be honest with you. The more honesty you get, the easier it is to determine the consequences associated with the actions.
Misinformation can be defined as willfully giving misinformation by commission (lying) or omission (remaining silent). Any student who is deemed guilty of misinformation is subject to disciplinary action which will be weighted according to various factors. Those factors can include the significance of the misinformation, the reason for providing misinformation, and/or the admission of such.