Assignment of Staff. One or more child care teachers shall be assigned to each group of children as needed to meet the requirements for group size and staffing set forth at §§C—E of this regulation. Age of a Child. For purposes of determining group size and staff required under this chapter, the age of a child is determined solely according to the date of the child's most recent birthday. Same-Age Groups. In a group of children in which each child is younger than 2 years old, the following staff/child ratio and maximum group size requirements apply: In a group of children of the same age who are 2 years old or older, the following staff/child ratio and maximum group size requirements apply: Mixed-Age Groups. In a mixed-age group with infants or toddlers, the following minimum staffing levels apply: In a mixed-age group with preschool children: The group size may not exceed 20 children; If the group contains preschool children 3 years old or older, the staff-to-child ratio is 1 to 10; If the group size is 13 to 20 children, the group may not contain more than six 2-year-olds; and If the group contains children who are 2 years old and the group size varies, the following minimum staffing levels apply: School-Age Groups. In a group where 3-year-old and 4-year-old children, enrolled in a public or nonpublic school, are mixed with school-age children, the following minimum staffing levels and maximum group size requirements apply: Group Size and Staffing in Approved Educational Programs. During a school day approved by the Department, a teacher shall be assigned to each class and the following requirements apply: A nursery school may not exceed a staff/child ratio or group size requirement set forth at §E(1) of this regulation, except that a Montessori school that has been approved by the Department may exceed a staff/child ratio or group size requirement by no more than 1/3. When children who are 2 years old, 3 years old, 4 years old, and 5 years old are grouped together, the average age of all the children in the group as of September 1 of that school year is used to determine the group's maximum size and staff/child ratio.