Samantha tells Tabitha's teacher, Mrs. Peabody, that "this is Tabitha's first school experience". However, the earlier episode I Don't Want to Be a Toad, I Want to Be a Butterfly (1968), was Tabitha's first school experience with the same teacher, who was then called Mrs. Birch.
The character portrayed by Maudie Prickett is called Mrs. Peabody. In the end credits, the character's name is listed as Miss Vogel.
Samantha didn't need to stand at the classroom door and hold it open to check on Tabitha. She could have just peered through the wall. She also could have disappeared when the janitor walked in on her at the terrarium.
Doing any of those things instead of standing at the door, would have involved using magic which she didn't want to do.
Doing any of those things instead of standing at the door, would have involved using magic which she didn't want to do.