Seriously...
I know it's just a movie and most scary movies are like this but seriously, if that thing was after me and it was on top of the bus, I would keep stabbing it over and over and over with the javelin. Because we all know that the monster is never truly dead when you think it is.
Like at the end when the dad kept stabbing him I was like. Finally, someone with some sense.
Even though it didn't work. I would still do it just to make myself feel better.