Schools like UPenn and Cornell (actually most schools) have divided schools. However, schools like UChicago don't. Does this mean that UPenn and Cornell students can hardly change their majors across different schools? For instance, a student admitted into the school of arts and science cannot take CS classes or transfer into data science major (which is mostly in the engineering school)
Correct. Its very challenging and difficult to switch majors at UPenn and Cornell. Say you want to get in Wharton or the Nursing school at Penn or the Architecture School at Cornell. It think it would almost be impossible to do that once you're admitted for arts and sciences. 1.) they are highly specialized colleges 2.) they have a special track you are supposed to be on from Freshman year and 3.) they tightly control the amount of cohorts in these schools. Now swithching out of a specialized school into Arts and Sciences is not a problem, the exact opposite. So if you are in the Architecture School at Cornell and decide its not for you, well they have such a low rentention rate anyway, that students have be switching to other majors for a 100 hundred years already. Plus getting into the Architecture School is exponentially harder. Same with classes. If you are Architecture Major, you can take a programming class CS no problems, but if you are CS student, they wouldn't let you take a design class ever. Does that make sense? I hope this answer has more of the detail you are looking for.
Good Luck.
It's going to vary depending on each university and the individual colleges within the university, but generally, students can change their majors across different schools - it'll just quite difficult (usually) and maybe impossible for certain majors. Students will usually be able to take classes in other colleges (they have to for some general education classes), but there might be restrictions for certain courses. Again, this depends on the university you're applying to, so check the school's website for the details.
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Oh and also, can I take engineering classes as an econ major? or I just cannot get a degree across different schools but I can still take the class? I'm so confused.