I love this show. I adore Lisa. I am a Lisa Simpson. However, episodes like this take me out of the reality of the show. Lisa is in second grade. Second graders can't tell time or count change. They can't write in cursive. Some can't tie their own shoes. Granted, I know Lisa is super-smart, but not THAT smart. I think sometimes the writers forget how young she is. I'm willing to give them some leeway--I suspend disbelief most of the time and just go with the story they're telling me, but this is too much even for me. She's still a small child. A second grader home alone can warrant a call to child services, never mind leaving her in charge of other people's kids.
This is one of my all time favorite series, and aside from this whopper, the rest of the episode is cleverly plotted (the brother/sister dynamic between Bart and Lisa is dead accurate), and the other characterizations are very well done (Maggie jumping around after eating coffee ice cream? Awesome!). I'd just like to see the show getting back to the basics--episodes like "Lisa's Substitute" where she's a real little girl are by far the best!