unless everyone bought or leased one from their bookstore

Aha. There we go. I work in higher ed.

To my knowledge, this is legal. Unethical, but legal.

I don't know if you have grounds for a lawsuit, but I highly recommend you begin recruiting students wanting to speak out against this. As an undergrad, when my school had a ridiculous hike in fees for a necessary item in one year, a bunch of us grouped together and started a campaign together. We ended up getting administration to negotiate, and they agreed on a tiered structure in the future to avoid huge hike like that in the future, and it lead to a happy medium for both sides. I've seen situations like this happen time and time again.

As long as you come up with a well thought out campaign, and conduct it in a mature, professional manner, you may be able to make enough waves to enact some change. Additionally, if you have a student government, it would be wise to collect the names of those interested in speaking out against this policy, and bringing it forth to your representatives who can bring this up at a Board of Trustees meeting.