Student Graduation Protests in Texas

A friend sent me a link to this article this morning.

There is a group of students and parents picketing some Forth Worth (TX) school district offices today representing the "613 Fort Worth seniors who did not pass the Texas Assessment of Knowledge and Skills exam."

I don't really understand why school officials are not allowing the students to walk in the graduation ceremony when they haven't graduated? Maybe it has something to do with the signs they're parents are holding?