It seems like there is a change of teacher issue.
I faced the same issue earlier this year when my elder went up to K1.
He refused school and cried buckets.
After i find out more, it's due to a change of teacher and circulum. For N1/2 is more of play rather then studies. Whereas K1, they focuses more on study and a little of playtime. He had a hard time to adapt. But now he is well and happy. In fact he loves studying