I'm pretty sure it's the subject that's mentioned most often.
OMG! Just realised something! History = Angel! He 'tutors her in history' and she fails - more than fails, she can't even write her own name... and after S2 she leaves Sunnydale - and her name - behind! ::is overwhelmed with cleverness of BtVS::

hadn't seen the parallels to Season 6 in it.
Neither had I until I went looking! And then it was suddenly very obvious. Poor Buffy indeed.