My son is traveling to Italy with his school will he need a green card and if so why [closed]

I presume you are asking because you have seen the green card listed as a document required for entry into Italy. If so, that entry refers to the international motor vehicle insurance card, and it is only required of people who are bringing a vehicle into Italy.

The other possibility is that it refers to a US permanent resident card, but that document is not generally required of people traveling to Italy.

All your son needs to enter Italy is a passport or EU national ID card. If your son is not an EU citizen, he'll need his passport and possibly, depending on his nationality, a visa.