after 3 teenagers are killed which exploring a school thats been closed for 20 yearsan aging cop decides to take on the case and he eventually realizes there is more to this case than he could ever imagine.

this is pretty sick i like this idea it could be good.

What makes this case personal to your protagonist? Why should I care if he STARTS this investigation, much less watch the film long enough to find out 'there's more to the case than he can imagine' - which is so obvious an element in any whodunnit that you don't need to waste words saying it. Your logline needs to grab your audience without having to state explicitly that there will be shocking reveals in the second half.

A retired detective must face the biggest challenge of his life by retracing the steps of three murdered students in an abandoned institution.
??