0) Don't slow down the class for the slower, unqualified kids. Pace it for the majority.
1) Make sure the kids know your expectations to be successful in your class the first day.
2) Identify a "path out" of the class for the unsuccessful kids (to Honors Physics?)
3) Have the first test early.

At our school we ask students to "Apply for the AP Sciences" through the
Science Department chair. There is an application that asks for their
report card, a copy of their standardized test grades and an essay on
why they want to take that AP class. Although we have the grades and
test scores down in the main office, this is a way to weed out the lazy
ones. Additionally, the essay weeds out the ones who really don't want
to do work. For AP physics, we also require their math teacher to sign
the form.

Although this process is not perfect, it certainly does a good job of
elevating the status of the courses and letting students know they are
one of the "selected" ones.