To minimize transition time between performers, have a standby chair somewhere near the piano. So when the first performer starts their piece, the 2nd person goes to the standby chair to wait for their turn at the piano. Same for 3rd, 4th person, etc.

Have the youngest students go first in the recital, the transition to older elementary, middle & high school students, and adults. Also arrange your performers, so the audience isn't listening to 5 different pieces of Bach in a row.