A teacher day --> This doesn't make sense. It could say 'The day of a teacher' or 'Teaching for a day' perhaps.

My job is a teaching from teenagers from 16 to 18 years old. My classes are is about science and maths. I have only/only have ten pupils in my class because it is/they are a special group with problems to approve in the normal secondary school -->Red bit doesn't make sense. My pupils are of different nationalities. As well as spanish, I have morrocon, Romanian, and ecuatorian nationalities in my class, all of different ethnicity. I love my job because every day is different than the rest and explaining the nature is awesome.

A teacher day

My job is a teacher from teenagers from 16 to 18 years old.

(I teach teenagers ages 16 to 18 years old.

My class is about science and maths.

(I teach Math and Science.)

I have only ten pupils in class because is a special group with problems to approve in the normal secondary school.

(I teach special education and so my classroom only has ten students in it. My students cannot learn in a regular education setting.)

My pupils are from different nationalities, also spanish, I have morroco, Rumanian, and ecuatorian nationalities in class, and different ethnies.

(The students come from diverse backgrounds. I have one student who is Spanish, one who is Moroccan, one who is Romanian,  and one who is Ecuadarian. They come from different classes, as well, as some are wealthy and others are not.

I love my job because all day is different than the rest and explain the nature is awesome.

(I love my job because each day is different and teaching Science is awesome!)