Prerequisite classes are most likely mandatory courses the school requires in order to be accepted in their school. Any class that is mandatory must be taken or listed on an AADSAS application. From what I understand, you will still be accepted to the school if the course is listed on your application, but you MUST complete the course before the dental school year begins. I am not sure exactly what happens if you fail to complete such a requirement, but I would imagine that the upcoming dental student may lose the spot in that dental school.
Dental schools also have recommended course lists. These are different than the required courses and completing them is optional, but highly recommended. Completing the recommended classes as as the required courses do improve your chances of getting in, but not taking them should not hurt your application. Taking these courses also puts one at an advantage during dental school. Many of the recommended courses are classes you will be taking during your first two years of dental school which is very similar to medical school. During these two years a dental student is taking around 30 units of classes at a time. Exposing oneself to the material during undergrad will free up time to study for other classes.
If you are still concerned about prerequisites, I would not trust anybody but an admissions representative from the school. From what I have seen, taking classes at a community college is okay but completion of the requirements at a four year university is heavily preferred. .