Many articles say the following:
"She is going to school."
"She is not going to school."
"It is not the case that she is going to school."
However, I think only (2) is the correct negation statement because (1) is only one of the scenarios (2) implies. For example, the case that another student is going to school also belongs to the (2). And there are many cases belonging to (2) such as the case that I, who is not she, is going to school.