The 100 Series by Kass Morgan series

First 3 books in series:
The 100
No human had set a foot on Earth for thousands of years.
The units of survived people live in a spacecraft away from the earth's surface which is contaminated by radiation after an overwhelming nuclear war
And the hundred of difficult teenagers, which the society said goodbye in advance, now have a dangerous mission to re-colonize the planet Earth. This task can be their chance at a new life, or - a sentence.

The Hundred 21
The hundred on the Earth for 21 days already. They are the only people who visited the planet in centuries...or as they thought they were.
Faced with an unknown enemy, Wells attempts to keep the group together.
Bellamy decides to save her sister's life, in any case, no matter what, while Clark goes to a Weather mount, in search of the other colonists
Gless has faced the unimaginable choice between love to life and life itself while being on the ship
So Kass Morgan's sequel "The 100/Hundred" knocks
in this rhythm, secrets are revealed, beliefs are challenged, and relationships are tested. And the hundred will struggle to survive, the only way they can – together

The Hundred Return Home
The fate of hundreds of troubled teens that were involved in a cruel experiment of colonizing the Land unoccupied for many years after the environmental disaster is tensely observed by the whole world.
The guys managed to create an oasis of order among the chaos reigning around in just a few weeks that hundreds spent on Earth. When lucky enough to leave the Colony units arrived from the space, where we're almost no oxygen the delicate balance was disturbed. However, the Glasses' luck seems to end there. Clark leaves no thought about her parents that may have survived and become one of the leaders of the rescue expedition to the wreck of the shuttles of a new batch of colonists.
Bellamy faces a choice: to escape from the camp or to answer for a crime, that seems to be left in the past, while Wells is trying to preserve his credibility, despite the presence of the Vice-Chancellor and his armed guards. Well
It is Hundreds' time to unite and join the fight for found on Earth's freedom, otherwise, they risk losing everyone they love.