So teacher is mwalimu and teachers is walimu? Why not wawalimu? Is it a different noun class or is it because the plural otherwise would have a double wa?
In m/wa class singular noun can have [m] or [mw] and plural has [wa]. Examples: mwanafunzi -> wanafunzi, mtoto -> watoto.
wa walimu would mean "of teachers " walimu means "teachers" while mwalimu means "teacher" becareful where you use it