Lana assumes that since the dates on the photos of her mother and the other man are after her parents were married, that there must be something suspicious about the couple. Dates on the backs of photos merely show when the photos were developed. They could have been taken years before.
Earlier in the season Clark tells Pete about his secret, however during the final scene with Chrissy, Clark hesitates using his powers around Pete and then distracts him by telling him to lock the doors in order to speed-run to confront Chrissy. This error is due to the episode originally being filmed for to air earlier in the season.
When Clark shows up to school and asks what's going on, the new principal states that the gate locks when school starts and doesn't open back up until school ends. But this doesn't make sense because right before this, Lana states that she has the first 2 periods off. How would her or other students that have the same type of school schedule as her, get into the school without the same trouble?
It's revealed that Chrissy has been sucking the youthfulness out of her victims since the 1920s, but all of the other "villains" thus far were made that way through some kind of contact with the meteor rocks. Of course, the meteor shower only happened 12 years prior to this episode. No explanation is given for Chrissy's rapid aging nor her youth-sucking powers.