Why is Ernest graduating such a concern for the segregationists?

The segregationists don't want any of the nine students to graduate, but Ernest Green is a particular concern as he's the oldest of the group. If he graduates, then they're worried that this will set what, for them, is a dangerous precedent. So, the opponents of school desegregation use a variety of stalling tactics to try and prevent Ernest from graduating such as filing legal challenges. More ominously, they resort to violence and intimidation against the students and their supporters. The upholders of white supremacy are getting increasingly desperate in their campaign. They are all too aware of the huge symbolism of a black man graduating from Central High for the first time, so they set out to do anything they can to stop it from happening.