Presenting is when a person's secondary gender fully blossoms, so to speak. Before this, no one knows what their secondary gender will be (though it is common to speculate). Presenting usually happens during or after puberty, typically beginning with the onset of a heat or a rut. Alphas know they're alphas if they pop a knot, whereas omegas will get wet. Betas don't usually... do anything. And that's how they know they're a beta.

During puberty, students usually go to health classes like IRL students during puberty. There, they learn about heats, ruts, blockers, etc.