Torrington's old biology teacher, Mr. Black, allows himself to be bitten by a spider, transforming him into a half-spider half-human creature to get revenge on the staff who forced him into "early retirement" because of the unorthodox/non-Torrington approved experiments he was performing in his lab. (The spider was really a spitting spider, which is why it is shown that Mr. Black was able to fire a web thread from his mouth.)

Also, Martin is completely thrilled and maybe they'll finally get some new blood (preferably of the female variety) to teach the class.