"They both work in school."
Translation:Oboje pracują w szkole.
Generally translating "both" is difficult because there are many options.
Just to discuss Nominative, you have "obaj" (two men), "obie" (two feminine nouns - so in this sentence two women) or "oboje" (one man, one woman). Also "oba" for two masculine/neuter nouns that are not people, but it's not an option here.
Now, if you use "oboje" (one man, one woman), you can't use "one", because "one" can't include a man. So as I wrote above, in this sentence you can use:
Either "Oni obaj" (they both are men), "One obie" (they both are women) or "Oni oboje" (one man, one woman).