Mozart's "Alleluja" K.553 (listed as K,553) is a delightful choral fugue that showcases his mastery of contrapuntal writing and joyful expression. Its intricate vocal lines and clear harmonic progressions make it a staple for choral study.
Focus on precise articulation and clear voice leading in each of the contrapuntal lines.
Alleluja K,553 is rated Henle level 2 of 9 — beginner repertoire, roughly ABRSM grades 2–3. In the RCM system it corresponds to about level 2.
You should be comfortable at Henle level 2 — meaning you can already play level 1–2 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Alleluja K,553 lasts about 1.5 minutes.
Alleluja K,553 is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.