Was it obligatory to conceal the female student after she informed her colleagues about preliminary acts of fornication, and does not concealing her expose the one who did not conceal her to Allah's not concealing her on the Day of Resurrection, given that she knew the student had not ceased her actions?

The student should be advised to abandon forbidden acts and their propagation. If she desists, then conceal her matter. However, if she insists, or if it is known that advice will not benefit, then reporting the matter to the school administration is correct, in order to curb her mischief and rectify her morals or to expel her. For the one who openly commits a sin should not be concealed, and this includes one who speaks about what they secretly did.