"Where is the teacher?"
Translation:Mwalimu yuko wapi?
-po, -ko and -mo are locatives, think of them as the location version of "to be".
Saying "Mwalimu ni shuleni", doesn't really make sense, though people will probably understand what you mean. The sentence "Mwalimu yuko shule" isn't perfect either, as there is no preposition. So a better sentence would be
"Mwalimu yuko/yupo shuleni" (yupo implies a more specific place than yuko)
"Mwalimu yupo mbele ya shule" - The teacher is in front of the school