You just have to quickly hide behind another predator once one as noticed you, the same way you force a pred to swallow a student. Just be careful, because you have to be quick enough so that one pred see you and the other pred that you are trying to get swallowed dont have time to grab you. The easiest way I found was at the start of a "two random pred" game, run straight to the cafeteria (where both preds spawn). They always spawn facing the kitchen and there is often one pred behind the other. So go in front of the pred that is the furthest from the kitchen until they notice you, then quickly hide in front of the second pred, since you need the second pred to be between you and the first pred. Don't be to fast though, because the second pred might swallow you first.