I have had a look at these pictures again.And I must state I was wrong.These Ju88s in the pictures are different planes.The white 18 numbers are of different shapes.
The backside writing of my picture proof that the tailnumbers are not used only by schools.
Writing on the picture:
Nach unserem ersten Einsatz Castel Bonito (Tripolis) 15.10.1942 (After our first sortie Castel Bonito (Tripolis) Oct. 15th 1942
Thanks Jan! I tried finding that book, or just a preview of it rather, but can't! I don't want to buy a whole book for what would literally be one piece of information.