Corrected entry: When Paige and Leo are talking in Leo's office at magic school, Paige compliments Leo on being such a good headmaster. She mentions that after being headmistress for a year, it's harder than it looks. If she became headmistress after they killed Gideon (i.e. when baby Chris is born) and she's been headmistress for a year, that would make Chris at least a year old. Chris is obviously not one year old just yet.
Corrected entry: When the sisters visit the vampire queen to make a deal, Piper blows up several of the vampires in their bat form. But in season 4's "Bite Me," the book of shadows says vampires are immune to witches' powers.
Correction: The Book of Shadows states that vampires are immune to witches' powers while in vampire/human form but are able to be destroyed in their bat form, which happens in both season's 4 & season 7's episodes.
Correction: Seeing as she is working at a school, she probably meant a school year, which only lasts for 10 months and not a whole year. She took over for Gideon after he died, so the school year had already started. And she also argued with the Elders first on even keeping the school open, so the school year was already partly over before she actually became headmistress. So like I stated previously, she probably meant a school year instead of an actual full year.