It isn't just that the level of education of this school is high. It's that it's also been consistent for years.   

A The level of education in this school, which is usually quite high, shows only slight variations from year to year.  

B The standard of education is not high in this school, but at least all the students are at the same level.   

C It isn't fair to deny that this school is successful, as it has had the same high standards for many years now.    

D Not only are the standards of education good in this school, but it has maintained those standards over the years.