DD7 went through a similar experience. At 4 she had a wonderful preK teacher who recognized DDs abilities and helped her explore them. As a result, DD had a great attitude towards school, and had lots of friends.
Then came K. DD was getting shoved aside, bored, frustrated, etc. She started pretending to dumb down in order to fit in. She started showing a lot of the same symptoms you're reporting here... pretending not to know things she'd mastered long ago, openly stating she hated learning, collapsing anytime she was offered anything that wasn't easy, etc.
The solution was to homeschool for that year. Now she's back in school for 1st grade, which is still not up to standards but at least is better, and the same can be said for DD's attitude towards school and challenges.
