My three kids go there. Their website has all the info you are looking for. You can get the info in German and in English. Kindergarten has bigger classes (18-25), 1st grade has between 16-20 (that fluctuates as kids are coming and going), second grade has about 18 and the upper grades have less, since the school is still growing.
They just moved to a new location in Al Mamoura (after they were located in 2 villas) next to the blue water tower.
www.ds-doha.de/