<p>There should be, but people use them interchangeably. What you (and I) think “admittance” should mean is called “enrollment” if you want to use the term that is both precise and conventional. </p>
<p>So we say that colleges accept or admit students, and then some percentage of those students enroll, and those are the ones who actually attend the college. “Acceptance rate” and “admission rate” invariably mean the same thing. Sometimes, though, colleges will use the term “admitted class” to mean the same thing as “enrolled class”, but I think the latter is gaining ground as the preferred term.</p>