My good mistakes are...

1-“San Pedro Nolasco is a catholic school where there are no foreign students like in the most schools in contrast to the USA reality.”

“San Pedro Nolasco is a catholic school where there are not foreign students like in most of the schools, in contrast to the USA reality”

2-Here we can find the most common environment in this kind of institution, there are few troublemaking students that bully shy pupils but they are not the most.

Here, we can find the most common environment in this kind of institution; there are few troublemaking students who bully shy pupils, but they are not the most.

3-The students belong to a low social class, most of them do not have economic resourses enough to pay for a private school...

The students belong to a low social class; most of them do not have enough economic resourses to pay for a private school...

4-It is very helpful to be part of one of this group because interaction is the most important thing to improve fluency and to gain accuracy.

It is very helpful to be part of one of these groups because interaction is the most important thing to improve fluency, and to gain accuracy.