How important are senior mid-year graders for high-achieving students, really?

Most colleges are using 9-11.
Colleges will often reset your gpa to their calc and will weight with honors /ap.

Where it may have an impact and I don’t know - is a school like u of Arizona. With a 4.0 unweighted you get $32k in merit. With a 3.9-.99, $30k so $2k less.

Will the grade make any college in America change their mind about you? No chance.

I doubt most at even the top ranked schools have perfect grades.

You are worried for nothing.

Good luck.

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