How many letters does the top answer for “used of competition or cooperation between secondary schools” have?

The most common answer is interscholastic (15 letters).

Crossword clue: used of competition or cooperation between secondary schools

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

interscholastic 15 letters wn-def

used of competition or cooperation between secondary schools

interschool 11 letters wn-def

used of competition or cooperation between secondary schools