I don't think nonresident foreigners qualify to attend public school nor would an education visa cover these. A private school may be a different story.
At bare minimum, providing it's possible:
1) A visa for the US
2) Payment for schooling
3) A place for the child to live
4) Someone agreeing to cover living expenses and take legal responsibility for the child
5) Healthcare coverage
6) Power of attorney for the caregiver
Now if the child showed up at the border claiming to be from El Salvador, then maybe anything is possible as he gets gobbled up into the migrant snafu - you may not be able to get him back.