THE RULE:
c'est / ce sont + noun Ce sont des élèves.
il est / ils sont + adjective Ils sont sympas.
c'est + introduction to a person, place or thing
il est + description
She is my girlfriend.
She is social!
They are my friends.
They are not funny.
That is my math teacher.
They are independent factors.
She is different.
He is an important man.
The test? It is not hard.
She is really selfish.
They are very polite and respectful students.
He is a brilliant scientist.
That's Mr Martin.
He is her son.