Nice to hear from you again!
From my personal experience of going to school in Berlin, being able to go to Gymnasium depended on my grades and not only on one teacher.
Unfortunately, these school procedures can vary according to the federal state/ Bundesland. According to the source I found, there are some Bundesländer where the parents' decision counts and not the grades (in Niedersachsen, Rheinland-Pfalz). In others, a certain GPA (grade point average) decides if the child will be able to apply for Gymnasium (e.g. in Bayern, Baden-Württemberg). And sometimes, the parents who don't agree with the "Schulempfehlung" (school recommendation) can send their children to something like a "testing class" (Probeunterricht) to see how the children are doing in the new environment (e.g. in Berlin, Nordrhein-Westfalen).
Maybe the experts from LernLaden Neukölln @BildungsberaterIn can help with this case?
All the best,