Mr. Hackensaw is the antagonist of Gregory McLeod's School Flash-animation as part of the Halloween 2005 special. He is a mysterious, murderous man with a low temper who is tall and thin, wears a black shirt and pants, has brown hair, and wears sunglasses.
In School, Mr. Hackensaw appears as an evil substitute teacher for a class of five students in Regal Valley High School. He is first introduced by the principal as the class's substitute after the regular teacher, Ms. Sunnyfeld, had been involved in an accident (whether or not she was killed by Mr. Hackensaw remains a mystery). He appeared briefly in this Halloween special to teach a small class of 5, in which he managed to kill each one. He is the guardian of the class because The only lesson he taught was "IR>U".
Mr. Hackensaw has exhibited a few magic abilities which, although not fully explained, are thought to be due to him being a demon. He has dark magic which can stripe victims of their flesh in seconds, the ability to send people into other dimensions, and the ability to pull weapons out of hammer space. He also can turn his eyes red.