Get the actual degree requirements and actual course schedules from some
 undergrad school you are considering, along with the entrance
requirements for a dental school you are considering.  Then write a M-F
8-5 schedule for 2 to 4 years of undergrad work.  It is a lot of work,
but the best way to get a detailed view of what has to happen.  Have
your high school adviser help you with it.

Do a similar exercise for an Optometry degree or PhD in Biology or
whatever might interest you.  By comparing the different schedules, you
will have a good idea of your options and decision points.

Hi, my name is Neil, and I live in Canada. I am entering my senior
year, and I'm struggling with what degree should I take after high
school. A pre-dental degree or a bachelor of science degree? The thing
I like about pre-dental degree is that it takes shorter time to
complete(2 years) than bachelor of science(which takes 4 years to
complete). Let's say I am halfway completing my pre-dental degree and I
decide that dentistry isn't the career for me. This might sound like a
dumb question, but is dentistry the ONLY career you can do with a
pre-dental degree? Because with a bachelor of science there is more
variety available.

My other question is whether or not is it possible to complete a
bachelor of science degree in two years? If you take courses in the
summer, is it possible to complete the bachelor of science degree
earlier than the recommended time to complete it?