There is no single ideal weight for children of a particular age, especially since they are continually growing and changing as part of normal development. Growth and development happen at different rates for different children. Restricting food intake might negatively affect their development.
As a teacher, your best approach is to encourage all students to make healthy food choices from the four food groups and to be active every day. If parents express concern to you about their child's weight, encourage them to talk to a Registered Dietitian (RD).
Visit dietitians.ca to find an RD in your area.