Do you look at high school results from undergrads

If they would have done that with me I would have been screwed. I suppose if you are younger they might. However, if you have solid performance during your 4 year I do not know why they would give a shit. That being said, getting into a good school is not going to be easy if you have amounted to piss in high school.

Nope, I've never even had my high school's name on my resume. The college you attend is usually considered a proxy for your high school performance, in the sense that if you sucked at high school, you're probably not getting into Harvard.

Your ACT/SAT will come with you from high school but literally nothing elase

They might ask about your SAT or ACT. Some firms requires this on your resume. I have never heard a firm formally asking for your high school grades or a transcript. If you are a freshman or sophomore interviewing for an internship it could casually come up in an interview, though even this would be rare.

In the UK, your high school grades are used as a screening mechanism for pretty much every big name internship or grad job application. Bit sucky for late bloomers but that's that.

Generally your high school performance dictates your school for college so it is generally easy to see if you were a huge fuck up or did well enough and matured through your years at college