"....to help teachers design their educational strategies to build upon correct ideas."
When you 'build upon' something you are adding something extra. It is like when you commence to build a house. You put in the foundations of the house (concrete slab, perhaps), then you 'build upon' or ADD to that base .... building the framework or the rest of that house.
You can also use this idea of 'building upon' or 'adding', to abstract ideas (plans, strategies, for example).
I create the beginning of this plan/strategy and then I ADD to it ('build upon it') by developing or expanding the plan further with additional ideas or strategies.