During the first 24 to 48 hours the child should stay home from school and take antibiotics for the conjunctivitis "Pink Eye".
After 24 to 48 hours of taking antibiotics then the child's pink eye shouldn't be contagious and can go back to school.
Usually a child can go back to school within 24 hours but I would wait 48 hours to be sure the child isn't contagious with his/her pink eye.