Reason is simple.

I has NO idea what course ID to use since it is not a child of
anything when you open the form independently.

If sites is really independent of courses and/or School, then it
should be a separate table of JUST sites and what you select in form 2
should be a table that contains the courseID and the selected Site id.

If site is entirely dependent on course then it should never be
created except in a situation where the system can know what course is
involved.

Ron