When I did a search there were no stories about Severus Snape and Katie Bell and I thought "Oh, how tragic!" so here is my take on one:
Katie was walking down the hallway towards here next class which was squeezing the puss out of boils on toads. Katie liked this class as she got to drink the puss afterwards and she thought it tasted yummy. She also took this class as she wasn't good at anything else and when she grew up she wanted to be a toad's boil puss squeezer. She would even get a badge.
As she was walking down the hallway she ran into someone. "Oh golly gosh," said Katie, "You must be a rather large person to have gotten in my way. Are you an elephant?"
She looked down to discover that she had in fact run into her favourite potions teacher professor Snape.
Katie immediately went bright red as she realized that she was wearing her top that said "I Love Professor Snape!" on it.
Professor Snape said that he could never love Katie as she would make him wash his hair and he could never do that. Instead he ran off with Crookshankes and they got married and Katie wasn't invited to the wedding.
She was so sad she ate her television.