My comment has no application to the Suriano situation, the validity of which I don't think can be questioned, but this isn't an accurate generalization. The list of valid affirmative defenses to contracts that otherwise meet the above-stated criteria is looong; the majority of time spent in a 1L contract law class concerns those exceptions to facially valid contracts.
Many of those exceptions are based on the legal presumption that the parties are "arms-length" bargainers. For instance, a minor is almost never an arms-length bargainer with an adult, which is why such contracts are void by law. And there are contracts of adhesion, where one party had no meaningful ability to bargain (a take it or leave it contract); there's mistake, impossibility, lack of acceptance, lack of consideration, failure to disclose key information relating to performance, ambiguity, and many many more.
Again, not saying any of the above are in play in the Suriano situation. But the first two I mentioned, incapable minor and contract of adhesion, is echoed by the imbalance in sophistication between schools and student-athletes as legal parties to a contract. These contracts are drafted in school-friendly terms, reflecting that imbalance. And the terms don't differ much from school to school--the realistic alternative to acceptance is not signing any contract with any school.
A contract of adhesion isn't invalid on its face, but it's halfway there and courts are warier of them, depending on the context. I don't really doubt the validity of the school/student-athlete contracts, though, because (a) I haven't meaningfully researched them; and (b) they're time- and litigation-tested (I assume), legal vulnerabilities should have been purged by now.