I think you're getting a bit mixed up here. For example, I went to Harvard, which has a large endowment which has grown substantially. But the endowment definitely didn't grow by moving tuition money into the endowment (it goes the other way around, with parts of the endowment helping to fund financial aid). Endowments tend to grow through donations and hedge-fund style management.

It may be the case that some universities are pulling less from their endowments to cover certain costs, covering those costs with increased tuition, and leaving the endowment free to grow more quickly, but I'm pretty confident that universities aren't just siphoning money from tuition straight into the endowment.

I think you'd have to show that universities are spending a smaller fraction of the earnings from their endowments, rather than just that the endowments are growing.