The school mystery

This story story starts with 2 students who are both girls there life was normal like any teenagers going to school meeting up with friends... Until they were sent a email by their school to go on a Saturday morning they didn't think much of it but then they got there they only saw some other students who they recognized them from some other classes so they ask where are the others only to found out that those students also got the same email as them while talking they heard a scream so they runned in different directions in result to lose each other but after they closed the doors they can't open them in result they have to do some tasks to find the keys to open the doors and at the same time to be careful to not get killed will they find out who is the killer with the mask? And who send them those emails? And last what is the reason of them being there?