After experimenting the challenges of teaching basic musical education and trying to figure out how I learned it, I figured out that the main problem might be the lack of clarity about how to practice it.
This picture shows the main scheme for intonation. This means: the goal here is to get 1) pitch right and 2) the names right . But I add still a key component that most methods forget: to get a feeling of the intervallic structure and degrees of the scale right! For this we have the numbers there.
The slur helps to know where to sing. The exercise can be made with fermatas or not. But if you sing in tempo, you must be musical.
Only after you are musical, you can go to the next level of difficulty.
This should be used interactively in lesson in diverse ways.
The teacher can hide the name of notes, or ask the student to sing leaps starting from C. Details about how I use this I can give later.