I wonder if anyone wants to over-analyze this joke to come up with the best candidates for the number? 57 falls too easily to the quick divisibility by 3 trick, IMO. (I realize over-analyzing jokes always makes them unfunny but we can pick the best number here and nobody has to know).

Me! I'm in!

Most-nearly-prime number under 100 is.... 91.

13 x 7. You've either memorized that or you're going to have fun deciding it's not prime. And if you did times tables up to 13, congratulations, you're special.