Before you begin prepping for so & mi, students need to understand the basic concepts of high and low pitch - they need to be able to compare and contrast two notes and work out which is higher and which is lower. So how do we teach this?
I like to begin with songs that expand their vocal range and pitch-pattern the music in a nice subtle way. We're not yet asking them to consciously identify high and low, simply to copy our movements. Songs such as "Flea" and "We're Going Up" are great for this, as both start with touching your toes and working your way up to the top of your head - moving with the pitch of course!
Then, once you have made conscious the idea of high and low pitch, reinforce it with some fun games! I've already written about one of my favourite high/low games "Apple, Apple, Ripe and Round" in this post but some other favourites include Andy Pandy and "Reach Up High"
Introducing students to Art Music that features high or low pitch and having them respond with bodily movements (eg. grow like a tree with the pitch) can be a great multi-purpose activity. You're expanding their repertoire of music, developing their active listening skills AND building a strong kinaesthetic connection to high and low pitch. Some examples I like include contrasting works like "The Elephant" from Saint-Saens' "Carnival of the Animals" or one of the bird sections from Tchaikovsky's "Peter and the Wolf".
You can also further develop students' understanding of high and low pitch in conjunction with their singing voice development by allowing them to perform the high and low pitches for themselves. You can utilise a variety of activities such as puppets, string, pictures on the whiteboard and many more ideas found in last week's post about vocalisations.
Later on, when students have made conscious so & mi and are preparing to learn how to put these notes on the stave, all these concepts and activities are worth revisiting to once again strengthen that visual/aural/kinaesthetic connection and understanding of pitch.