SJS:
You are missing out a crucial point in the tuition calculations.
Most families on tuition assistance are aware that they will NEVER be able to pay tuition.
So they give up. It goes out of the equation. They pay (or try to) whatever the school asks of them, and they live their regular life with what they have left. They have no thought of attempting to pay more in tuiton, as the feel “since I will never get to full anyway, what is the point”.
Are they wrong? Probably, if they would ask their Rav, he would tell them so. IIRC Rav Schwab was Makpid not to accept donations from those parents on tuition assistance.
Will it change? Also not. Everyone believes tuition is way too high already, but most families have no choice. (Teaneck is different, as I will explain in a minute.) So they go on scholarship, and do what they want with the rest. But to punish the child is a bad idea, as they will resent the school and Torah for it.
Teaneck, on the other hand, has the option of JFS (I think) in SI, which makes the only excuse of sending to the “Legacy Schools” as they are called and not paying in full vs. paying in full and going on a 45 minute bus ride (boo hoo:(, laziness.
If you really wanted to change things, you would have to give a larger disincentive to go on scholarship. Back in the day, it was a Busha to have to give over your tax records to the school. Now, in the days of MOFES, no one cares. The schools need a new disincentive. And withholding the report card is not it.