Similar to the original topic here, 15 seems to be rather young to be making a decision about what you’ll do for the rest of your life. What happens if they change their mind and want to go to University? Can students move back over to that path if they want to?
...it is a little complicated.
typically kids get sorted into gymnasium, the type of public school leading to "abitur" (which is the main way to get into a college) after year 4 (bavaria, my home state) or year 6 (berlin f.e.). in bavaria you need certain grades to get into gymnasium (leading to college, f.e. jobs like doctor, teacher, lawyer...), certain (not as but nearly as high) grades to get into realschule (less demanding, leading to vocational training for mostly business or office jobs after year 10) and everybody else attends mittelschule (more practical stuff, leading to vocational training after year 9 for jobs like hair dresser, car mechanic...). (there is also lots of special education stuff for kids with specific challenges; usually they go to special schools, some attend regular school with assistence.) parents can choose which school their kid should attend but only if the grades are there or an entrance exam is passed (which happens not very often; the kids passing tend to give up during year 5 or 6 at gymnasium because it is academically too demanding for them, they can not keep up). teachers are trained very differently for these different types of school.
there are also private schools (state funded up to 80% of their budget) but only if they have a special profile, like religios school, montessori, waldorf and the like. they usually have fees.
these can chose students on their own but can only do their own "abitur" test after year 12 if they adhere to the state grade cuts. if they do not their students have to test afetr year 12 at a public school near them and usually (at least very often) these kids fail or do very badly even if they pass.
so let's say you attend "mittelschule", went into vocational training to be a doctor's asissent after year 9, passed the vocational exam after three years. you are now around 18 or 19 years old and want to go to college. you now also have a certificate which is aequivalent to those of the students who finished "realschule" after year 10 successfully. this gets you into a public school which leads to a simpler version of "abitur" after 2 years. you can then go on and attend a "fachhochschule", these are collees specialising in more practical, less academic studies. typically they produce social workers, engineers, lots of different things. you can also stay another year and then pass general "abitur" and after that you are free to attend any college you can get into (colleges choose their students. some take anyone, some have very specific entrance exams, others select depending on abitur grades...; but never money decides). of course you have to be able to produce the necessary academic work. many are not. which is no problem, they are are already trained professionals.
this is only one example how one student could get into college if not being selected for "gymnasium" at age 10. there are many more. in general you can and will get into college if you want to, work hard and have the necessary academic talent even at age 15, 18, 30, 50 or 70 or whatever.