I Ain't Gonna Spray Lettuce No More - S4-E3
Continuity mistake: At the beginning of this episode, Cory asks Alan for money for his class photo. When Alan asks what grade Cory is in now, Cory says 11th. However, at the end of the previous season, Cory had just finished 9th grade (he and Shawn are both 15) and he spent his summer traveling the country with Eric. He should be in 10th grade, not 11th. This is further complicated later in the season, when he turns 16 and gets his driver's license. If he was in 11th grade, he would have already turned 16 and gone through driver's ed.
Answer: Topanga's middle name is never specified. Considering her sister's name is Nebula Stop-the-war Lawrence, Topanga's is probably something equally unusual.