Senka no Kanojo

From MangaHelpers:

It's the last day of their school trip, and Yuki finally gets his courage up to ask out the girl he likes, Makoto. Very soon afterward, the two of them learn how fragile their world really is, as a missile attack bombards the city. Many are dead and injured, and the newly dating Yuki and Makoto must help each other endure the shock and grief. They try to meet up with other survivors from their school, with the goal of getting home safely to Hokkaido. But could this attack be, as predicted online, the beginning of the "last war"? [tethysdust]