The only requirement for admission is that one be a witch or wizard of the right age.
“No, I am a teacher,” said Dumbledore. “I have come to offer Tom a
place at my school.”
“What school’s this, then?”
“It is called Hogwarts,” said Dumbledore.
“And how come you’re interested in Tom?”
“We believe he has qualities we are looking for.”
“You mean he’s won a scholarship? How can he have done? He’s never
been entered for one.”
“Well, his name has been down for our school since birth —”
—Harry Potter and the Half-Blood Prince
Recall that Tom Riddle was born in a Muggle orphanage to a very poor mother—not exactly the sort of birth likely to be reported to the Ministry. And indeed, Hogwarts does have an automated way of ensuring admission. The Quill of Acceptance writes the names of all wizarding children in the Book of Admittance upon their birth. From an interview:
In Hogwarts there's a magical quill which detects the birth of a
magical child, and writes his or her name down in a large parchment
book. Every year Professor McGonagall checks the book, and sends owls
to the people who are turning 11.
This also indicates that only those who will turn eleven before the school year are allowed to attend. That subject, however, is sufficiently covered in the other answer.
The approval of the Quill and the Book is the only criterion for acceptance to Hogwarts. As written by Rowling:
In a small locked tower, never visited by any student at Hogwarts,
sits an ancient book that has not been touched by human hands since
the four founders placed it there on completion of the castle. Beside
the book, which is bound in peeling black dragon-hide, stands a small
silver inkpot and from this protrudes a long, faded quill. These are
the Quill of Acceptance and the Book of Admittance and they constitute
the only process by which students are selected for Hogwarts School of
Witchcraft and Wizardry.
More specifically, the Book often demands proof of magical talent:
A mere whiff of magic suffices for the Quill. The Book, however, will
often snap shut, refusing to be written upon until it receives
sufficiently dramatic evidence of magical ability.
This is the only admission test. One's relationship with Death Eaters, the Ministry, or indeed the Headmaster or Headmistress is irrelevant.