Several parts combine into one character.

級 is a phonetic-semantic compound: 糸 (silk thread) + 及 (jí, "to reach / extend to" — providing the sound). The composite reads as "ranks distinguished by silk-grain texture" = grade / class / rank. The metaphor is from textile craftsmanship: silk fabrics were graded by the fineness of their thread weave, and that hierarchical sorting became the model for ranking learning levels, athletic competitions, and social classes. Mainland simplified to 级.

Korean reading "geup." Saturates Korean educational and competitive vocabulary: 高級 (gogeup, high class — used in product-quality marketing: 고급 호텔 high-class hotel), 上級 (sanggeup, upper level), 下級 (hageup, lower level), 初級 (chogeup, beginner level — used in language and skill courses), 中級 (junggeup, intermediate), 階級 (gyegeup, social class), 體級 (chegeup, weight class — boxing and wrestling). 급 is a key counter and adjective.

Mandarin jí, 2nd tone (simplified 级). 级 (jí, grade / level), 一年级 (yī niánjí, "first-year grade" = first grade in school), 高级 (gāojí, advanced level), 级别 (jíbié, rank), 班级 (bānjí, class — academic). Mandarin uses 级 actively in school vocabulary: students reference each other by 几年级 ("which grade?"). 等级 (děngjí, rank / class / category) is also high-frequency.

Japanese on-reading キュウ (kyū) — 上級 (jōkyū, upper level), 初級 (shokyū, beginner), 中級 (chūkyū, intermediate — common JLPT level vocabulary: 上級者 jōkyūsha "advanced learner"), 階級 (kaikyū, class). Japan diverges from China on the grade-counting word: Japanese uses 年生 (~ nensei) for school grades — 一年生 (ichinensei, first-year student), not 一年級 — though the kanji 級 still appears in vocabulary like 学級 (gakkyū, classroom group / homeroom). Japanese martial arts and arts use 級 (kyū) as the rank below 段 (dan, black belt level): 三級 (sankyū, third-rank below black belt).

Memory aid: silk threads (糸) reaching out (及) — graded by fineness of weave became all kinds of ranking.