An escalator school (エスカレーター学校, esukareitā gakkō) (also esukareitā kō (エスカレーター校) and esukareita kō (エスカレータ校)) is a school that offers education from elementary or middle (or even from the kindergarten) until university. Escalator schools are so called because students usually rise to the next grade without having to take entrance exams. While many Western private schools are this way, escalator schools are far more prevalent in Japan than in other countries.
Fictional examples
Escalator schools are commonly found in anime and manga, as they are a quick and easy explanation for realistically having characters of very different ages in the same school setting.
References
Escalator school Wikipedia(Text) CC BY-SA