Have I ever said she was going to completely forget everything she's learned? No. Am I basing what I've said on a lifetime's experience of watching classmates have to relearn material after extended time away from studies either over the summer, or because of illness-related absences and stuff? Yes.

You're right that she had no idea what was going to happen when she went off with the Doctor. As in she had no idea what was going to happen. The choice is either that she made the conscious (or subconscious, take your pick) decision to ditch her studies for however long it struck her fancy to do so and try to pick back up where she left off whenever she returned, or that she didn't even contemplate the notion that there might actually be consequences to her actions.

Look, I could argue with your lot until I'm blue in the face. Clearly anyone speaking in the least bit negatively of Saint Martha is just going to get dumped on, so I don't even know why I'm bothering.