As @cathnog said, kluk == boy, and kluci == boys. Croatian, also a Slavic language, has a sound change called "sibilarizacija" (assibilation). This means that in some cases "k", "g" and "h", when in front of "i,"will change to "c", "z", and "s", respectively. I can't find the rule for Czech, but I believe it's at least similar, if not the same. It can sure be applied to this case: kluk + i = kluci. Now let's hope some native Czech speaker replies and confirms this...
i am a native speaker. But this is a rule that is very hard to confirm exactly because I am a native speaker. You do not think of stuff like this when it is your native language. It just is :D
That said, every masculine ANIMATE noun I can think of that ends in K in singular ends in CI in plural.
Žák = Žáci (pupil, student) Novák = Nováci (common last name)
INANIMATE change to KY
Mák = Máky (poppy) Hák = Háky (hook)