Top schools compete with each for students. Collaborating to keep prices high or keep admissions criteria consistent in a way that reduces competition between the schools would open them up to anti-trust complaints.
But that would be hard to prove. If two gas stations collude to set prices, that violates anti-trust law. But if they just follow suit as they see the other raise or lower prices, that's generally legal, as long as there isn't a secret or understood agreement. I expect the latter is more what's going on here.
But for there to be an antitrust case, consumers or suppliers must have been harmed in some manner to the benefit of the trust. Standardized testing costs are pretty low (especially compared to tuition), and they don't reach the colleges/unis.
But that would be hard to prove. If two gas stations collude to set prices, that violates anti-trust law. But if they just follow suit as they see the other raise or lower prices, that's generally legal, as long as there isn't a secret or understood agreement. I expect the latter is more what's going on here.