If you want a GREAT horror movie, go buy Troll 2. It is by far the most incredible film ever made. If I was a horror movie director I would have given up when this was released, you just can't top it.
The R18 certificate is given to hardcore porn films which are designed to sexually arouse the viewer. Although I haven't seen Teeth, from what I've read about it it doesn't sexually arouse viewers; therefore, the R18 certificate isn't appropriate.