Rebecca, Chad's girlfriend, was mentally cruel to you, rather then physically, and 18 in. Soft is admittedly a little bit of overkill, so you figure she must pay too. As your third class starts, you set it back to as long as Chad's change, the whole class period.
Rebecca, at the start of class, instantly feels something wrong in her tight, grey yoga pants, but tries her best to ignore it. 3 inches in, and it's clearly noticeable, so she hides her textbook on it. At 6 inches in, she begins to hear the fabric stretch, as it can no longer hold her very long and thick dick, with an appropriately sized balls as well. She stands up to run to the bathroom at 8 inches, but it's too late, and her yoga pants explode open, exposing her new, sensitive dick to the class. Running to the bathroom, the stimulation causes it to harden to over a foot long. Satisfied, you make it so that all changes made were permanent, head home for the day.