Wanton describes something excessive, uncontrolled and sometimes even cruel. The principal sees a food fight as a wanton act of vandalism done with wanton disregard for the rules, but the kids might just see it as fun.
Wanton comes from the Old English wan- "lacking" and togen "to train, discipline," in other words, "lacking in discipline." Wanton extravagance is excessive and uncontrolled, whereas a wanton act of terrorism is random and intentionally cruel. Sometimes in older novels, you will see wanton used as a disapproving term to describe someone viewed as promiscuous, a usage that is considered old-fashioned today.