Niner is a rite of passage ceremony marking entrance or acceptance into a group or society in High School, required for grade nine students to be fully "accepted" into the student community.
Another word that could be used for this term is Initiation. However niner is a more specific term referring only to grade nine initiations.
This type of initiation is mostly done in school trips that require students to sleep over,(for example; Muskoka Woods camp) and at night around 1:00 to 3:00 AM, grade 11 or 12 students would wake up, and go around "initating" niners in their sleep.
This term is a new type of initation, started in Bishop Allen Catholic High School, in Toronto, Ontario, Canada.
The following could be considered ninering, having your hair sprayed into a vivid color, usually green, having shaving cream sprayed on your face, or make you do ridiculous things (if not in a school trip.)