Maybe (if your goal was to get them to score higher) you could have pitched it as gambling for entertainment, i.e. record which answers you guessed on, and later compare who was luckiest in getting the biggest portion of those correct.

Most apathetic students have no qualms with guessing. It sounds like these peers of yours are either extremely diligent and motivated by fear, or unwilling to lose face by admitting that you're more clever than they.