Translation:The teacher will speak about the rules of the school.
school is not a real possessor, it doesn't own the rules, in English too you'd say school rules and not school's rules. It's a compound, the first name acts like an adjective while the second name takes the genitive suffix. I give you more examples:
diş doktoru= (tooth doctor) dentist
senin diş doktorun= your dentist
güneş gözlüğü= sun glasses
senin güneş gözlüğün= your sun glasses
you see that the glasses belong to you not to the sun!