Formula to number child records as related to a parent record, based on date

This solution will work on a going forward basis, not for existing lessons.

You can do the summary of the # of lessons and then look that up down to the Lessons table.  Then use a snapshot field https://help.quickbase.com/user-assistance/setting_up_snapshot_fields.html to freeze that value

The lessons will be numbered.

However, if you delete a lesson record, the numbering will end up double up on an ordinal.  ie this will usually work but is not a perfect solution and also will only work on a going forward basis.