Follow the chart below, EXAMPLE - the foot measures 20cm, check the comparison size with school shoe size (size 13), If you want a firm fit choose size 13, If you want growing room go up half to a full size depending on age (13.5 or 1).
It is not recommended to go up a size if shoes are needed immediately for exams.  
Choosing width: If you're average/wide choose 'C', if you find your foot to be average/narrow choose 'B', size A and D are for the extremes in narrow and wide.

Measurement of the foot: Please measure sitting down with the foot off the ground, use a ruler and measure from the heel to the end of the big toe, or longest toe. (standing will flatten the foot and give a false measurement).
This chart is a guide only and should not replace a personal fitting.