Rinkai Gasshuku

The protagonist, Sanada Naoto, transfers to a new school at the worst possible time, right before summer vacation.
His classmates are all eccentrics, and they suddenly become attached to him on first meeting, accuse him of things he didn't do, and he even reunites with his childhood friend...

And so, he has to go to an overnight seaside training camp with this group.

Masato doesn't want to go, but his cousin encourages him to.

"But you know, this training camp might actually be interesting."
"If you spend days with new people on a trip, you'll see things you never thought you'd see..."

And so an unforgettable summer began.