Hi! I just got Valedictorian for my sim. I hope this can work for you too.
First of all, sentense from messages above that your Sim should be first to achieve A and maintain it -- is wrong. Well of Desires put my sim to B grade at some point but he earned his A grade back and became Valedictorian on aging.
Things that were done right before and on that happy Monday:
1) A grade.
2) Well passed exams on Friday (but aging on Friday went without Valedictorianship, so that's not enough.)
3) Completed school project on weekend. I bought it by myself in Buyld-Buy.
4) He was wearing jewelry with simanite (looks like this did biggest part), emerald (can be good also) and amber (doubtful; I used this because in my language "sim becomes more lucky" were in description).
5) Went to school under my control.
6) He used potion of concentration before each class (logic and natural science; I suppose this did second biggest part).
Afrter first class his "event perfomance" was full on two bars out of three. (Good and relevant sign, I think)
I get notification that he is best student.
Then I made him age up without birthday party and he get Valedictorian. Reloaded and ran birthday party. Still get Valedictorian.
My Sim had all school skills at level 10, topped all after-school activities and maxed all careers.Quit all of them several days before.
I think, Valedictorian is really about hightest schol performance at some point and some luck. But simanite jewelry (and may be emerald) rise your sim to unreachable levels so no other student can compete. (But amber still can be a thing. Can't guarantee that.)
School project also rise school performance bar, so completing one should be given a try.
And few mood potions won't hurt.
I hope this will help you finally get Valedictorian.