The primary reason high schoolers started first was to allow more time for work sports and watching little brothers and sisters.   Does it always work out that way definitely not.   Say even 10% of the older siblings watched younger siblings.  That changes childcare costs substantially for those 10% of families from free to hundreds/thousands of dollars a month.   As said younger siblings would need to be watched for 2+ hours until the older sibling could watch them if they would even release the child to a non adult.