Unfortunately, a lot of people are trying to get away with paying, which is wrong. I once served on a tuition committee. A few examples.
1. A son works for his father and shows an unusually small amount of income on his 1040. Looking at his lifestyle it is obvious he is paid off the books.
2. A person who I knew had switched jobs in the past year shows his w2 from 1 employer with a reduced annual income instead of his 1040.
3. A person who just remodeled their home, had a maid and wanted a reduction.
With people like that, unfortunately the Yeshivos have to be tough.