Quite often teachers are worried about the fact that marimbas are diatonic – The AMI standard C marimbas have Fs and F#s – but don’t have any other chromatic notes. Of course, this limits the number of possible keys that the instruments can play, and some music will (sadly) be impossible.
The majority of the marimbas being played around South Africa do not have this chromatic option – and so the music they play uses only the few keys available from diatonic C and G. The energy and enthusiasm you see in these bands suggests that the players are anything but bored. The inherent rhythmic interest of marimba music with its syncopated patterns and vibey basslines sustains the interest – the emphasis is on groove, not harmonic complexity. One result of rhythm based music is that students become confident at improvising rhythmic riffs and they develop excellent ensemble skills. The harmonic possibilities of songs are less complex, and so the focus stays on the rhythm. This could be one of the reasons most bands learn to play by ear successfully – the limited pitch and harmonic options simplifies the learning.
If you do want the extended melodic and harmonic potential of a fully chromatic instrument then there are a few options available. In South Africa, Marimba Workshop makes instruments with removeable notes (in the style of Orff xylophones), and this facilitates different keys – you just change a note for it’s chromatic alternative and bob’s your uncle. Some schools like this option. AMI chromatic instruments are fully chromatic and their sturdy design facilitates playing. The diatonic and chromatic sections are firmly connected, so there’s no problem with your chromatic notes shifting.