<p>As SnowflakeVT says, schools typically accept more students than they have places for because they know that not all accepted students will matriculate for a variety of reasons. Many applicants wind up with more than one acceptance and have a decision to make. Others may not get as much financial aid as they had hoped for, or may have a change of heart about going into a performance program at all.</p>
<p>Many music schools do have wait lists, some implemented in a formal manner and some more informally. Schools like Curtis and Juilliard are in the position of having very high yield rates and of being able to find highly-qualified students who would be willing to transfer in on short notice. For example, I have heard of a case where an international student who had been accepted to Curtis found out that they could not leave their home country at the last possible moment. Even though the start of the semester was only a few days away, one of the teachers who also happened to teach at Temple asked their top student there to transfer into Curtis and the vacancy was filled in a matter of hours. (I suspect that Temple, although a fine music school, was not able to attract a replacement as quickly.)</p>
<p>Different schools have a variety of ways of trying to smooth out the process so that they do not wind up with either too few or too many of a particular instrument, but even so it does not always work out. Some use rolling admissions and throttle back on the acceptances as the year plays out. Some have early auditions in hopes of securing at least some commitments early on in the admissions cycle. Some maintain formal waiting lists in the same manner that a non-music school might and send out a second wave of acceptances after May 1 when student decisions are due.</p>
<p>Even so, things sometimes do not work out the way a school expects. If a school that has a yield rate in the more typical 25% to 40% range accepts four oboes in hopes of getting one or two, and it turns out that all four matriculate that year, then they may not accept any the following year. This is one of the reasons that auditioned programs are never a sure thing and that it is important for the student to find out how many spots really are available in the particular instrument and particular year in which they apply to a school of interest.</p>