Under state law, charters — which are privately operated — are entitled to a “reasonably equivalent” share of space on public school campuses. The Los Angeles Unified School District says Magnolia already occupies its fair share, and though the district could choose to provide more space, it won’t — for reasons officials have not clearly explained.

Nowhere are the challenges and tensions of such forced collaborations more acute than in L.A. Unified, which has more charters — 225 — than any other school system.

Access to district campuses is important to charters because land and construction costs are prohibitive. And the competition for students has become especially intense as overall enrollment in L.A. Unified has declined, threatening the viability of many schools. New charters continue to open — their growth funded by philanthropists — and they now enroll nearly one in five district students.

Los Angeles Unified has sharing arrangements at 80 campuses, more than any other district because of its size and because the California Charter Schools Assn. repeatedly has pursued litigation to enforce state rules.

But one school’s success often is seen as being at the expense of the other. Parents and teachers at numerous schools have led unsuccessful protests to keep a charter off campus. At some campuses, the district has gone so far as to demolish outdated and outlying buildings, which increases playground areas while also deterring charters from claiming available classroom space.

To school board member Richard Vladovic, putting a charter on an unwilling L.A. Unified campus almost never pans out.

“What I’ve seen is animus,” said Vladovic, who represents the Carson area and recently began a one-year term as board president. “I think it’s real bad for kids.”

The complications run big and small.

Magnolia has to throw out unopened milk every day because it doesn’t have access to refrigeration in the cafeteria.

The district-run school no longer has a band, but if it wanted to revive the program — which is in line with district goals — it would have a problem. The charter has the wing with the music rooms.

Magnolia has a band but objected to the district’s designation of a music room, with built-in risers, as its classroom for disabled students, because some of them have mobility problems.

To work around that, the charter swapped access to the library for possession of an empty space that used to be a weight room adjacent to the gym. That room has its own bathrooms and the floor is flat.

The two principals are cordial in managing day-to-day issues, although Magnolia Principal Shandrea Daniel has a list of things she’d love to improve. Her assigned “science lab,” for example, has only one sink and lacks such built-ins as Bunsen burners and an eyewash station.

And the small central area between Magnolia’s classroom buildings is where Curtiss keeps its dumpsters….

At first glance, Curtiss Middle School, with a drab two-story main classroom building, doesn’t look like much of a prize. But with nearly 20 acres, both programs have access to an expansive grass field — a rarity at many district campuses. The children from the two schools stay separated in their own halves and use the gym at different times….