I think it was in 4th grade, the whole class for one of my kids got sent home a note from the teacher begging us all to start making our kids use deodorant! She said the smell in the classroom after recess was actually making her lightheaded :) If you can smell it, so can others, including folks who have to be in close proximity to your child.
We made our kids use a deodorizing body soap (AND WASH UNDER THEIR ARMS) daily plus use deodorant (which is not the same as antiperspirant) starting in like 4th grade. The boys used Arm & Hammer brand - which seems to do a good job for an excellent price point. The girls were always fussier but I remember purchasing Secret a few times. The only thing for me was they had to use stick and not an aerosol (personal preference).
Good luck!