You've confused two completely different conjugations, which is exactly why you should think of them as "conjugations" and not "prefixes," even if the writing of the word only changes by one letter. The vocalization of the word, which isn't expressed in letters, can change the meaning of the word. The conjugation "maf3al" or "mif3al," where "f" is the first letter of the triliteral root, "3" the second, "l" the third, means "place of" or "tool used to do." That's the conjugation مطار and مكتب are in. The conjugation "muf3il," or "mufa33il," or "mustaf3il," or any of the other conjugations where the meem is followed by an /u/ sound and the final consonant is preceded by an /i/ sound, forms agent nouns, like English "singer" or "gamer." That's the conjugation معلم is in. "ma3lam," spelt exactly the same way but pronounced differently, means "landmark," as in one of the sights that tourists frequent. Same thing with مدرسة, which depending on whether it is pronounced "mudarrisa(-tun)" or "madrasa(-tun)" can mean "teacher (female)" or "school." Since both are common words, I'm not sure which one you intended to use as an example.