I believe ZIS Baden campus will be shut down and the children merged back to their other campus if not already done so.
In Zurich there is also Intercommunity (ICS) and a number of other bilingual schools, including three with Montessori programmes for primary school and one (Lipschule) offers a differentiated/individualised programme for age 12-16.
Even with the best recommendations you are advised to get around and visit each potential school. The issues are far too varied.
If you cannot afford or want to attempt to integrate your child, then each individual area has their own local schools and there will be a big variation in wat is offered. They will need to do intensive language classes, and I would say that even for the brightest and most dedicated student (which adolescents normally are not), it will take 2-4 years at least to get the language to a confident level to thrive...and that is not taking into account the social challenges...
My eldest did 4 years (9-12 years) in the Montessori bilingual school and is just about to finish the local secondary school in the A level class, and has applied to go to the FMS (Fachmittelschule) to get in, she needs to pass an exam in German, French and Maths. She is a highly capable student who performs badly under pressure and is considered 'gifted' in languages...she is finally getting the hang of the concept of 'studying' now... At 15.5 years old..
Our second has complex medical needs, but did 6 years in bilingual Montessori and is also in the local Sek A, and doing reasonably well despite having erratic attendance due to ill health since he started secondary. Unfortunately, puberty affects his health condition, and it is making it highly complex, he is also 18 months behind in physical growth, so is the smallest boy in the class, which will either make him a target for bullying or a firecracker...it's going to be pretty complex to balance his needs, but so far the local school has been highly supportive...other than very worried...
The big 'plus' side of the Swiss system is flexibility with entry ages. Acceleration is frowned upon, but we have a friend whose son is 17 and entering his second attempt at gymnasium this year... If he completes the short gymi, he will be 21 going to university. Time will tell whether that is the best option for him.
If our daughter goes to FMS she can do a 3-4 year programme, finish at 19 then decide whether to do the teaching course that she has her eye on, or something else at the Fachhochschule (applied sciences/pedagogical university) or do a bridging to get to a classical university... Or go back to Australia and continue on there...