Dua after Salaah

Q: It is mentioned that a person's dua will be answered if he makes duaa after the fardh salaah. Does this mean that he should make dua after the fardh of the salaah or after the entire salaah is completed?

Jumuah in a school

Q: I work in a school, o​n Fridays we do Jumma salaat with jam'at.  Some people have said that our Jumma is not correct because (a) the general public can't attend the salaat (even though any staff or pupils who wish to pray are able to attend). (b) There are not enough people (we usually get about 10 people besides the Imam). 

My question is that is our jumma salaat correct or is it better for us to pray zohar namaz with jamat instead. What is the minimum number of people we need (besides the Imam) for the jumma salaat to be correct?