Will one plagiarism strike in sophomore year significantly lower my chances?

I’m a senior applying to college this fall. I had a plagiarism incident (in which I had a few plagiarized sentences that was a part of my notes that I had to turn in) in my sophomore year. My teacher told me that this will go on my school record, but not my permanent record. Do colleges look at the school record? Regardless, I’m worried about how this will affect my chances. I have about a 3.9 weighted GPA and a 1510 (out of 1600) SAT, fwiw.

You need to talk to your GC about whether this will be reported or not. No one here can tell you. If it is reported, it could affect your chances. The usual recommendation is to include more safeties & low matches than you normally might to be sure you have options in the spring.

Ask your GC.
And if you look at the application, you’ll see how the question is worded. Both you and the GC answer.