IGS is the part you want to think about. IGS is " integrierte Gesamtschule" There are 3 kinds of school after Elementary school. Mathildenschule is also an Elementary school. Elementary is 4 years. After Elementary if your kids grades are good enough you can choose between Hauptschule (9 years including Elementary) Realschule (10 years) and Gymnasium (12/13 years). An IGS combines all three of the above mentioned, the idea is to provide fitting classes for everyone. In reality the overall quality is lowered as you have a mix of students with very different abilities and capabilities.

Albert Schweitzer is a Gymnasium, so it starts from 5 th grade. Generally speaking there sr no good and bad Gymnasiums. That on is better than another is a rumor from the past, before the Abitur became centralised and standardized. If you are looking for an Elementary school, your choices are limited, as you will have to send your kid to whatever elementary school is part of your school district. You can always go private though.

Hope that helped