We know that by employing the formula for the average here, we can find the average number of tickets sold per school child as the ratio of the total number of tickets sold to the total number of children in the school.
Let A be the average number of tickets sold per school child.
Let T be the total number of tickets sold.
Let C be the total number of children in the school.
Then from the information provided in the question,
T=tickets1+tickets2+tickets3 and C=class1+class2+class3
Thus, A can be expressed as:

The above is the expression for the average number of tickets sold per school child.