The phrase "from a public school" is correct and can be used in written English.
Example: I learned Spanish from a public school in my hometown.
I came from a public school in Pennsylvania.
A lady volunteered to take 17 children from a public school to the Bronx Zoo.
He bought the gun less than two miles from a public school, he said.
Much of Maynard's early support came from a public school across the street from the Weeksville houses.
And they believe their companies and employees benefit from a public school system that has made measurable improvement.
"I'm a young African-American teacher who came from a public school education, from an urban environment," she went on.
Members of a team from Baruch College Campus High, a public school in New York City, drew inspiration from their daily subway commutes, when piled-up trash on the tracks caused train delays.
This next image came from Kikla, inside a public school now used as a staging area for fighters.
Here's one package from the Massachusetts College of Art and Design, a public school.
I resigned from my job as a public school teacher.
