I don't think everyone agrees that students need more of their shitty education. Honestly, if you can't teach it in ten months, you can't teach it in eleven months either. Poor educational results are a thing of standards, not lack of time.
Despite possibly being a remnant of an agrarian past, you have to look at what's more useful. As countries that favor creative thinkers over merely competent automatons, education during summer vacation may be more effective than a month of extra school.
Mind you, sitting at home playing PS4 is a terrible way to spend summer vacation. Travel, organizing and playing real-world games, exploring, hell, building a trebuchet or something, these are all educational, and can't really be done in school.