My nephews parents want to send him to Georgia from Missouri to live with me, I want to know if I should apply for guardianship or will notarized paperwork do for school and medical issues, I will be supporting him myself.

Notarized papers are nothing really, it only means a defense if the parents tried to file charges against you, Unless you have legal custody or guardianship of the child though the courts, you can not legally enroll them in school. You can provide emergency medical care for them, with proper medical forms The fact is, that the child is due support payments from the parents and you need court documents that the child is with you legally.

The parents should complete and sign a temporary guardianship form. They can get one online. Have that notarized. That should be sufficient.

... Unless you have legal custody or guardianship of the child though the courts, you can not legally enroll them in school. ...

Depends. As the school what it will accept.

The parents should complete and sign a temporary guardianship form. They can get one online. Have that notarized. That should be sufficient.

It really depends on the jurisdiction. Many will not accept a Notary but will require a legal change of guardianship through the court system.