How would you explain use of elbows

[rotationslim]

HS Rules.. player playing rough using elbows to move people out of the way.. I run a Church league I say you can't use your elbows like that.

He says "I need to use my elbows to get position under the basket"

Give me a good answer/explainer on what is allowed and not allowed under the basket for establishing position... so I can explain it to my volunteer refs..