Не съм сигурен за вашите данни и схема, но ако up.profile_value е като "123", можете да опитате това:
SELECT u.name AS _user_name, s.name AS _school_name
FROM fwg_files AS f
LEFT JOIN users AS u ON u.id = f.user_id
LEFT JOIN user_profiles AS up ON up.user_id = u.id
LEFT JOIN school AS s ON CONCAT('"', s.id, '"') = up.profile_value
