Usually, songs intended for K-2 are too long. This does not seem to be a problem with songs published for older students. Generally, songs of four phrases of four beats are appropriate for primary grades, songs of eight four-beat phrases for middle grades, with longer songs being appropriate but not necessary, for upper elementary grades. However, adolescent singers should not sing extremely long songs or for long periods of time. Most songs have repetition of some sort, such as tonal patterns, rhythm patterns, or text.
More repetition is needed for younger students than for older students.
The easiest type of repetition is when the words, melody, and rhythm repeat. A melody that repeats with different words is a bit more difficult but does not usually cause any problems for the students. However, when words repeat on a different melody, the students seem to have some difficulty, especially in K-2. Try to avoid this type of repetition for younger singers, and do not become frustrated if they have difficulty with this. Students need to sing songs in a variety of meters.