In the shadow of downtown Los Angeles’s futuristic skyscrapers, students huddle - often more than 50 to a class - in cramped, dilapidated Belmont high school.

Serving a staggering 5,410 pupils, Belmont is one of America’s largest schools. During breaks, students file along its gridlocked corridors, mirroring the traffic clogging the freeways outside. To communicate across the vast inner-city campus, staff tote walky-talkies.

Belmont’s 252 teachers feel lucky if they have 41 students in a class - that’s average. Space is so scarce that one-third of teachers rotate classrooms. This term, maths teacher John Zunino shifted rooms hourly during a five-hour block of lessons with the same pupils.

The numbers make discipline a nightmare. “A certain percentage of delinquents on a small campus is manageable, but with the same percentage on a bigger campus, you lose control,” said assistant principal Michael Summe. Belmont, which cut eight teacher posts this year, is one of many LA schools gripped by the overcrowding crisis that is demoralising staff and causing thousands of students to slip through the educational net.

At nearby Virgil middle school, a feeder to Belmont, 1,200 pupils aged 10 to 14 gather outside each morning awaiting buses to ferry them to outlying schools requisitioned to handle inner-city overspill. However, the school is ill-equipped for the 2,800 children spared the two-hour round trip. Many balance books on their laps as there are not enough desks.

While LA classrooms are bursting at the seams, nationally there are 23.8 pupils per secondary class (1999-2000 figures). Roughly half the states recently discussed or enacted legislation to cap class sizes. Florida earlier this month limited high-school classes to 25.

California has spent $7 billion since 1977 on reducing average infant classes to 20, but experts said this has been achieved at the expense of older pupils. In LA, the problem has been compounded by $78m (pound;52m) worth of job cuts brought in to ease a $450m budget shortfall. Its teachers are deserting in droves. There is a gaping hole in Belmont’s staffroom between newcomers and veterans, said Mr Summe. Many older teachers have few other job options.

All have to juggle overflowing classrooms with stringent new test requirements while trying to teach some of America’s most deprived and educationally backward pupils. These are mainly from poor families recently arrived from Latin America and 95 per cent of them do not speak English at home.

In the maths and English high-school leaving exams this year respectively 76 per cent and 59 per cent of pupils failed, and Belmont has no hope of complying with a new edict mandating it to offer extra coaching to these pupils. It is already at full stretch, operating year-round with staff and students divided into three staggered tracks of four months on, then two months off. Testing, made compulsory by White House and state reforms, ties up at least 18 days a year. Those who make it through to the school-leaving exam are the most motivated - 1999‘s intake of 2,000 14-year-olds has dwindled to just 618 17-year olds this year.

Such grim statistics are no reflection on staff, said English teacher Roger Peterson, who reckons 80 per cent of his pupils are from broken homes. But public scapegoating of teachers has left morale at rock-bottom, he said, adding: “Philosophically, a lot of us say, ‘I’m going to do what I can with the students and ignore administrators trying to work us into a testing frenzy.’ ”