You should only use "for a school" if you want to stress that they were your employer (e.g., when talking about being employed in the education system). Otherwise, use "at a school" ("in" is unnecessary, since most people work inside buildings). We use "at" to talk about the company or type of work (e.g., "I worked at Microsoft for two years" or "I worked at a factory in southern China").

You can use both at/for. 'at a school' refers to the job itself. 'for a school' indicates you being hired by the school to work for it as an employee.

In this case, you can use either preposition. It does not change the meaning.