“小学六年级课外”可以用“Extra-curricular for Grade 6 in primary school”或者“After-school for Grade 6 of primary school” 来表达。
“Extra-curricular” 侧重于指学校正常课程以外的、与学业相关或不相关的活动或学习内容。例如：“Extra-curricular activities”（课外活动）。
而 “After-school” 则更强调在学校课程结束后的时间里所进行的活动或学习。比如：“After-school programs”（课后项目）。
在具体使用时，可以根据上下文和想要表达的具体含义来选择合适的表述。例如：“小学六年级课外的英语学习很重要。”可以说成 “Extra-curricular English learning for Grade 6 in primary school is very important.” 或者 “After-school English learning for Grade 6 of primary school is of great significance.”
