Public Schools Require Proof That a Child Has Been Immunized

Public schools require proof that a child has been immunized against several diseases before enrolling.Why?

A) It serves as an entry barrier and so screens out children who are harder to educate.
B) It is a costly to fake signal that a child is healthy.
C) Since most children attend school,it is an efficient way to publicize the need for immunizations.
D) Failure to be immunized imposes external costs on other children and teachers.