Duet for each instrument is a good practice for sight-reading and also an opportunity to listen to your own sound (woodwind instruments often cannot hear their own sound well in an ensemble).
"Selected Duets" published by Lebank (now Hal Leonard) is a collection of duets for each instrument (flute, clarinet, saxophone, cornet, horn, trombone (euphonium), and the first volume of each (each instrument has two volumes, Vol.1 and Vol.2) contains many duets that even elementary school students can play.
Also, the cornet (trumpet) instruction book "Arban" contains many duets. It may be a little difficult for elementary school students to play on the trumpet due to the range, but it is a fun collection of songs to play on the clarinet.
It can now be downloaded from the IMSLP Petrucci Sheet Music Library homepage. Segment 4, edited by Goldman, is a collection of melodies and duets.
For band ensemble, Bach Chorales for Wind Orchestra (BACH CHORALES for BAND) has been published by Southern Music Company in the United States, and is a set of 28 chorales and 3 chorale preludes. You can purchase parts according to the number of students and the instrumentation.