secondary schools for kids Baden-Wüttemberg

Yes, there is compulsory schooling in Germany for 9 years (age 6 to 15).

But that does not mean that any individual school must take your kid. Contact your town's school authority ("Schulamt") and ask which is a suitable school for your child. There might be tests needed to determine this.

If he/she was in a bilingual German/Turkish school, it might be enough to forgo remedial classes for foreign kids (which are free of charge, but might cost him/her a year academically).

School is indeed compulsory if you now have residency rather than are just visiting. The question is how good is your child's German. Usually they need to review their situation for placement. Normally they would go to the closest appropriate school. If their German is not up to par, then they will need a special program to get it there and not every school might have this, or an open place. You need to contact the Schulamt, or local office of education rather than just an individual school. They will tell you which school the children need to attend.