How long ago did you start school?什么意思？

开学多久了

离你开始上学有多久了

病句
应该为how long have you been at school 或when did you start school
你上学多久了

病句,因为去掉ago,应该是How long have you been at school?意思就是:你上学多久了?
问多长时间不应用ago,ago是以前的意思,如果用上ago就是病句了!