For example, a form has 4 classes in Primary one and each class has 25 pupils. Hence, the total students in Primary one is 100 pupils. A form in Primary one can be divided as 1A, 1B, 1C, 1D. The top student in 1D might come 60th in the form i.e Position in class: 1, Position in form: 60/100.
The Hong Kong 'form' appears to be the British 'year'.