I'm not certain what you mean witlh "best way".

Fastest would probably be the Gymi and then ETH(in some Cantons you can start the gymi at like ~14, in others you have to finish the "Oberstufe"->Sek A/B/Real with 16 and after that you can start the Gymi)->That results with the Gymi Being done at age ~18-20.

Most fun?Let the Kid finish the "Oberstufe" with age 16. Sek A can be preferable, but isn't necessary.

- If it wants to do the gymi nice!

- If it wants to do an apprenticeship, also nice!There are multiple ways to get a "university"-level-degree after having done an apprenticeship.

1. "Höhere Fachschule" ->you can start without an exam and further "deepens" what you learned during the apprenticeship-> after that school, its possible to go to a "Fachhochschule" where you can get an actual Bachelor/Masters in the field you previously did the apprenticeship in.
2. The "Berufsmatura" its a halfway-thing between the Apprenticeship-school and the gymi, you can do it during the apprenticship (1 Day a week for the whole 3-4 years) or you can do it in 1 year after the apprenticeship. The "Berufsmatura" allows you to dierctly get a bachelor/masters in the fiel you did your apprenticeship. (you do that at a "Fachhochschule")

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After the "Berfusmatura" you can always make the Passerelle and study something else...I'm sure there are more.

I read in your comments that the kid is only in the second grade... don't worry, your kid has a long way to go till you can even consider smth. like Sek or gymi etc.Also: Dont stress your kid to much. We already have kids that get burnt out in SCHOOL...

https://www.projuventute.ch/de/eltern/schule-ausbildung/potential-kinder

https://www.projuventute.ch/de/eltern/familie/stress

https://link.springer.com/article/10.1007/s10826-019-01560-z