When does the next school year start for all round schools?

There's no single "all round schools" system. School year start dates vary widely by country, state/province, and even individual school district. To find out when the next school year starts, you need to specify the location (e.g., "What's the start date for the next school year in the Los Angeles Unified School District?" or "When does the next school year begin in Ontario, Canada?").