Elmbrook school district, located in california, held graduations at a local church. how might this violate the establishment clause?

Having a graduation at a local church might violate the establishment clause because the school cannot be associated with any particular religion.
There are many religions in the world today, and no school can "choose" just one to represent - it is either all or none of them. So, when this school held its graduation at a local church, it violated the clause because it opted for one religion before others, which is forbidden.