The Blue Letter (equivalent to "Pink Slip")

The blue letter is the colloquial term in Germany for a notification by which the school informs the parents of a pupil (or the pupil himself/herself if he/she is already of age) that the transfer is at risk.
Letters of dismissal are also often referred to as blue letters.

Even the Blue Letter itself is about to cry: The transfer is at risk.
© Photo by Paolo Chieselli at Pixabay
07.02.2022

In English this phenomenon is known as a "pink slip", but in Germany it is called "the blue letter".

The designation dates back to the 18th century, when royal orders often had to be packed opaque and sealed. Paper made from rags, often from Prussian blue uniforms of the time, was particularly suitable for this purpose.

Due to changes in the way blue paper is produced, it would have to be specially dyed nowadays, which means that blue envelopes are more expensive to produce and to buy. Therefore, at least in Germany, they are now rarely used for such documents.