Kelsey is sent to her new "Boston Comprehensive School". Lincolnshire still uses the selective school system so there are no Comprehensive schools. Based on her Eleven-plus results (or more likely her US grades) she'd either go to a Secondary school or to a Grammar school such as Boston High School.
Yeardsley mentions that they have to be in Stickney which is "some 40 miles away". Stickney is a village located about 10 miles away from Boston.