The second sentences here are not indirect instructions.
The first sentences are direct commands. The second sentences describe the resulting situation.
Sensei: 作文を書きなさい。
- "Write a composition!"
Indirect >> 先生は学生に作文を書かせました。
- "The teacher made the students write a composition."
Shachou: 鈴木課長、上村くんの世話をしてくれ。
- "Suzuki-kacho, take good care of Uemura-kun for me!"
Indirect >> 社長は鈴木課長に上村くんの世話をさせました。
- "The Shacho(company president) made Suzuki-kacho take good care of Uemura-kun."
The くれ part in the 3rd sentence is the command form of くれる, so you would be commanding someone to do something for you.
させる on the other hand means to "make", "force", "cause", "let" or "allow" someone do something. It's not used to express an indirect command in the cases you mentioned.
