I'm not too sure why Gakko III wasn't selected for the festival. It could be that it was too much of the same with its teacher-student-school trinity. Or probably the subject matter wasn't diverse enough for it to be differentiated for an audience already seen the first two, and still being fresh in the minds.
And Gakko IV - Fifteen, is very much different from the first two Gakko movies, in that it is not set within the confines of a school compound. Rather, this movie has legs, given that it's a road movie, and has its protagonist, 15 (obviously) year old Daisuke (Yuta Kanai) running away from home to go on the road, in search of a 7000 year old Cedar tree in Yakushima island.
Road trips stick to a formula, that you will encounter strange, quirky people along the way (that's what makes it interesting), and somehow have your lives touching each other's because Fate would only have it that way. You have the usual unfriendly bunch, and portrayals of those whom you will also like to meet up with, should you embark on a road trip yourself. Thumbing and hitchhiking his way, with little money and possession, Daisuke totally hates school, and this is something that any teenager could identify with.
But don't forget, Life, or living Life, is probably the best school there is. Throughout his journey, Daisuke meets up with people whom he will affect (for the better), and vice versa. The most memorable, and probably having the longest screentime devoted, will be his meeting with a woman trucker (Rei Asami), who, out of pity and curiosity, brings Daisuke home to provide him with temporal shelter and food. How Daisuke affected her family, with his bonding and bringing out the reclusive son, somehow worked wonders and cemented the theme of friendship in this story.
At the end of the day, personal relationships get healed, and one of the most touching moments was the expected run in with his family, especially his dad, at the end of the unannounced journey. That scene alone, with its relatively quiet moments, with subtle gestures, and almost muted feelings of regret, is very powerful indeed. That scene alone, is a worthy finale, and worthwhile in investing time to watching this.