Subordinate clause - missing comma

Question

I was wondering why there is no comma in some sentences containing a clause - for example:

A medida que salían de la clase les devolvíamos los móviles a los estudiantes.

Conforme vayan llegando los invitados ofréceles una copa de vino.

Accepted Answer

Hola Kelly
In Spanish the use of the comma is not compulsory in all clauses; it depends on the type of clause, connective, etc. 
What the rule says about this type of clause specifically shown in these sentences - these would be time clauses - a medida que..., conforme..., is that if the dependent clause precedes the main clause (this is the case) you use a comma if the clause is long, but if it is short you don't need it. 
I wouldn't say that the clauses here are too long or too short so we can say that here it is really optional. 
Saludos
Inma