是She is not good at swimming还是She does not good at swimming

be good at =do well in
这是固定搭配
你们老师是错误的
She is not good at swimming.
或者是
She does not well in swimming.
老师不一定总是对的，人无完人，谁都会出错。所以不要因为这一点而对你的老师有偏见；还有这一点也说明老师说的不一定都是正确的，所以要勇敢对你的老师讲出来。

be good at

恩 be good at

是be good at doing sth.
用do 的短语是do well in

She is not good at swimming.(be good at)
She does not well in swimming.(do well in)
我估计你的第二句话错了，"do"是个动词应该应副词“well"还有赶本就没有
do good at 的形式．

your teacher is retareded