In front of my house entrance a few tiles were drooping down, and I'm wondering what's easiest way to fix them, so that at least they won't drop and hurt anyone? I'm thinking about using screws but not sure if screws are strong enough to hold them.
I suggest you fix the leak first, or be sure it has been repaired. It appears in the top left of the photo. I do not know what the ceiling above it is so at this point is is a SWAG. If it were me I would get new tile and furring strips. Properly fasten the furring strips to the existing ceiling. Then staple the tiles to the furring strips. finally add the appropriate molding around the outside against the wall. The other option is to remove the existing tile and trash them. Using the appropriate glue you can fasten to the existing ceiling. Molding would be a nice touch.