What’s a smart way to manage ID checks for new users?

We recently had a spike in signups after launching a student discount campaign, and while it brought in a lot of legit traffic, it also attracted a bunch of fake accounts trying to exploit the offer. We want to avoid ruining the experience for real users, but clearly we need a better fraud filter. I remember someone here mentioned a system that adjusts how it verifies based on user behavior. How’d that work out in practice?