How long after you start treating a 2nd grader with pink eye before he can return to school?

Depends. What is the cause of the "pink eye"? If viral, then once the redness, itching and soreness disappear. If bacterial, it should be less transmissible after beginning antibacterial eyedrops.
Once healed. Pink eye is caused by the common cold virus, and is one of the most contagious diseases on the planet. And like a cold, the patient remains contagious until all symptoms are gone. The rule of thumb is that as long as the eye looks or feels bad, stay out of school. This can take up to two weeks. Also, since pink eye is a virus, there is currently no effective treatment other than patience.