The cell membrane is the outer boundary of a cell. It protects the cell from outside invaders, it regulates what enters the cell, and it regulates what leaves the cell.
I'm not sure how a school as a whole compares to a cell membrane. If the metaphor can be how a school is like a cell, then I can explain what part of a school is like the cell membrane. Many schools in my are are completely closed campuses. They have a fence that runs the perimeter of the campus (just like a cell membrane completely encloses a cell). It serves as a barrier to keep would be intruders off of the campus. It also regulates who can get on campus and when. During many hours of a day, the gates to campus are locked. They are opened at key times, which serves as a way to regulate movement onto and off of campus. That is the same with cell membranes and active transport. The cell membrane will contain special protein channels that allow transport of items through the membrane.