Here is the original item:
Because of all his responsibilities at work and
school,
(A) Michael
had no time to devote to the yard,
(B) so weeds,
vines, and insects thrived in this
jungle. (C) That
neighbors looked on with disapproval.
You wanted to do this:
Because of all his responsibilities at work and school.
Michael had no time to devote to the yard,
so weeds, vines, and insects thrived in this jungle. That neighbors looked
on with disapproval.
Adding a period after school will make a fragment. Because of all his responsibilities at work and school is a string of prepositional phrases without the necessary main clause that a complete sentence requires.
You might want to review the rules.