It is important to understand here the word "pre" in the context of colleges and universities and that means "preparation for"
Please see here:

Pre-Dental Hygiene
Preparation for Dental Hygiene School
What should I major in if I want to become a dental hygienist? With your advisor’s assistance you can design a program that meets the requirements to enter dental hygienist school. Students who have developed excellent writing and speaking skills, possess analytic and synthetic thinking ability, life-long learners who are enthusiastic, and understand the interpersonal aspects of medicine are the type of students most dental hygiene schools seek.

The “pre-dental hygiene” suggested plan is a beginning point for students. There are certain criteria that must be fulfilled for admittance to dental hygiene school. You and your academic advisor will need to examine your interests, your academic record and the requirements to determine the best academic path for you.

Early in your academic career you will need to research some of the dental hygiene schools you are interested in attending in order to insure you will meet all their criteria for admission

http://www.apsu.edu/cosm/predentalhygiene

and here:

Pre-Dental Hygiene Curriculum
The pre-dental hygiene curriculum prepares students to apply for the professional entry-level program; however, students may also transfer requisite coursework from other accredited institutions of higher education. Entry into pre-dental hygiene is open to anyone who is accepted as a student into TWU.
http://www.twu.edu/dental-hygiene/programs.asp

Just like pre-made also means preparation for medical school
Please see here:

Medical School - Preparation
Prospective Pre-Med Student Information
What constitutes a "Pre-Med" major at UC Berkeley?
"Pre-Med" is not a major. Students must complete the required coursework for medical school alongside the requirements for their intended major. Although choosing to major in Molecular Cellular Biology (MCB), Integrative Biology (IB) or another science is what many Cal students choose to do, you are encouraged to select the major that interests you the most. Medical schools do not prefer one major over another.
https://career.berkeley.edu/Medical/PrepPros.stm

and here:
Pre-medical (often referred to as pre-med) is an educational track that undergraduate students in the United States pursue prior to becoming medical students. It involves activities that prepare a student for medical school...
At most colleges and universities, students do not have the option of pre-medical major. A student on a pre-med track is permitted to choose any undergraduate major in any field of study, so long as certain required courses are completed
http://en.wikipedia.org/wiki/Pre-medical

Neither pre-dental hygiene nor pre-med are majors this is why your student is a major in Science/pre-dental hygiene. This means that while studying science she will prepare for dental hygiene school, which in many cases is under the school of dentistry.

Now: oral hygiene is usually translated as Higiene bucodental and also since "pre-higiene dental" or "pre-higiene bucodental" does not suggest anything in Spanish I think is necessary to come up with something that makes sense in the college/university context. This is why I am offering the option of "Preparación"

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Note added at 3 hrs (2012-08-12 22:24:48 GMT)
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pardon me I meant to say just like pre-med also means...