How to doonclick button one form to another form and come back to main form and if again open the other form it should open already opend form

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C#3.5

I have a main form contains three buttons one for School,another for College and another for Admin ,so when i click school form shows the school form and in that i will click on Home button, hide school form after open home form again i want to open School form it should open already previous open for(hide form) not new can any one suggest how to do this whenever i click on button it should not open every time new form it should open already opened form Using C# windows form

What I have tried:

I have a main form contains three buttons one for School,another for College and another for Admin ,so when i click school form shows the school form and in that i will click on Home button, hide school form after open home form again i want to open School form it should open already previous open for(hide form) not new can any one suggest how to do this whenever i click on button it should not open every time new form it should open already opened form Using C# windows form