'former' refers usually to something's state, i.e. something that was but is no longer. 'previous' refers to something occurring before something else (time or order). Both 'former school' and 'previous school' are grammatically correct, but I cannot help you with which one is more appropriate without the full sentence.

Former = "a previous school" but not necessarily "the previous school".

"Former" is more commonly linked to a person and "Previous" to a thing. For instance: "My former employee.." and "In my previous job..". I hope it helps.

former: a thing of the past; there was an unusual event that made it a thing of the past; there are usually only the former one and the current one; previous: the one directly before the current one in a natural flow/progress; there were many old ones before the current one, and there will usually be many next ones, too. former school: the old school in case you had to change the school because your family moved. previous school: the old school you graduated at

Please provide the full context. They are both good in thse short phrases.