Story highlights "We've been robbed blind," says a Detroit Public Schools parent

A vendor is accused of being at the center of the alleged kickback scheme

The bribery is said to have stared in 2002 and continued until 2015

(CNN) Thirteen current or former principals with the Detroit Public Schools system were charged with bribery Tuesday in an alleged kickback scheme, which couldn't have come at a worse time for the troubled school district.

In addition, a DPS vendor -- identified as Norman Shy, 74, owner of Allstate Sales -- was charged. He is accused of being at the center of the criminal activity.

Authorities say the scheme basically boiled down to this: Shy paid bribes and kickbacks to principals so they would allow their schools to be charged for supplies that were never delivered.

Chairs. Teaching materials. Paper. The very things that students need to learn and schools need to function remained, frustratingly, out of reach.

But at the same time the school district cried poor, principals are accused of having lined their personal pockets.

Read More