In On At (place)

         In

• to talk about locations:   

     There are a lot of heavy books in  my school bag.

• to talk about workplaces:      

     He works in a hospital.

• with class:

     She found it difficult to concentrate in class.

         On

• to refer to a position:

     I know I left my book on the desk.

• to describe a position  along a road or by the sea:

     Dublin is on the east coast of Ireland.

• to talk about a floor:

    They live on the 6th floor.

• to talk about being physically on public transport:

    I was on the bus when he called.

        At

• to refer to a position or location:

      He was sitting at his desk.

• to talk about locations at companies:

      Lisa is working at Microsoft.

• to refer to activities:

     Were you at Cameron's party?

• with school:

     I wasn't at the school yesterday.