What happens to Ichika after the fated summer ends?

Happy ending!!!

They have already told you about the ova, in chapter 12. The students were told that the movie was shot by former students and Ichica has the poncho of Bolivia. I think she came back a year after arriving, because they were in the first year and practically canceled the movie, as everything was recorded in the summer, they had to wait the following year (second year) and if they were already alumni, they would have spent at least three years (second, third and present year). Also, ... In the manga ....

We are given an OVA where you can see Kaito and his friends 2 years after the ending of the series. At the end of the OVA, you see Ichika crying tears of joy while she is looking into space. We can see that she is on a spaceship with her sister.

If we compare the ending of the series, where Remon-senpai build that ship and we can see Ichika in that sweater (or whatever) Kaito's sister brought from Bolivia after she left, with the information we get from the OVA, we can see that over 2 years had been gone till she came back

If you listen closely towards the ending of the show, one of the students said, that the movie was made by a graduated one this year. So, Kaito is now out of school, which means he ended 3rd class (2 years from 1st class till 3rd class). So, Ichika returned before he ended class (because of the movie, being published on the last day of school) but stayed about 2 years away, and now Earth's technology is at the point where she can stay and we have a happily ever after.

At the end of the last episode, Kaito's sister returns and brings a Bolivian sweater for Ichika, but Kaito tells her she had to return to her own country as her transfer had ended. We then see Remon at MIB headquarters with the plans for the interstellar ship and Rinon is there with her.

If you watch till the end of the credits, one year later, some students are watching the old class members' student film and it shows Ichika wearing the Bolivian sweater with some revealing dialog.

Ichika said that the earth was forbidden to be visited unless they had interstellar travel. In the end, we clearly see Remon with the schematics of Ichika's ship. This means that they must've made contact with aliens, joined the federation, and she could live with Kaito.