absent for "some time"
He's been absent from school for three days.
He's been absent from school a lot.
He was absent for three days last week.
from - This preposition can have the meaning of "away"
They are from Mexico, but they are in the US now. They are not in Mexico. (They are away from Mexico.)
The people of Mexico have a rich culture. - This does not refer to Mexican people outside of Mexico in the same way "They are from Mexico" does. It includes all Mexican people, mostly people living in Mexico but also those that are from Mexico (outside of Mexico).
not at school - away from school - absent from school
for - "For" is used for a number of reasons. One reason we use "for" is to refer to periods of time.
- The movie lasts for four hours. It's a long movie.
- They've been talking for two hours.
- We had to wait for three hours at the airport because of the bad weather.
- They've been rehearsing for two months.
- I can stay for a little while longer, and then I have to go.
- For Teachers