On one hand, just because person A has a university degree or two and person B does not, it doesn't mean that A is more intelligent than person B. Person B could be extremely intelligent, but did not go to university because of finances, personal goals and interests, or opportunities.
On the other hand, the level of education one achieves does have SOME correlation to intelligence. Dental school is very competitive; applicants generally need excellent test scores, so many people just don't have the ability to be admitted into dental school.
Therefore, while not being a dentist does not mean you're not extremely intelligent, being a dentist means you ARE very likely more intelligent than the average person.
Even if they ARE more intelligent than you, however, it is simply rude for the dentists in your office to treat their staff as if you are stupid idiots. That said, it may be valid for them to speak to you like you're below them, because technically, you DO work for them.