I think you owe it to the teacher, your child, and yourself to divulge that your child has ADHD. I too have an ADHD child and another that I highly suspect. My oldest is in 1st grade this year, she started Straterra just before kindergarten last year. I tell everyone I can that interacts with her that she is ADHD because we went through some pretty rough preschool years of being labelled as "disruptive" or the "trouble maker" and by other children as "the bad kid", and finally reached a point where I had a 4 yr old who had anxiety,low self esteem, and suffered stress headaches. This in turn impacted me because I fealt as if I had failed her as a mother.
For us kindergarten was magnificent. She was stimulated and busy; with her medicine controlled, she was able to sleep and be rested which helped improve our behavioral issues also. I don't advocate medicine for everyone, it has worked for her. My other daughter has started kindergarten this year and is not on medicine. She displays much of the hyperactive traits as my oldest and the "business" but we have had none of the behavioral issues. We constantly redirect and try stick to a routine that is consistent. I did inform her teacher of her sister's diagnosis and that I suspect she probably is affected also but not as severely, so that if she starts to have difficulty we can all,as a team, give her the support and the correct direction she needs vs. treating her as a "problem child".
The labels are hard, and they are even harder when you know that there is a reason behind there disruptions that frankly, a 5 or 6 yr old is not mature enough to handle without some adult intervention. I think you have made the wise choice and are being an advocate for a child who cannot advocate for themself yet.
It can be a challenge as a parent of a child diagnosed this early. I know I have fealt like it was all my fault at times. Some people don't understand and never will. I can understand your hesitancy to inform. I have been in the same situation with daycare and school. Will they be treated differently or scrutinized, or will they accept them at all (daycare). Honestly, I have found the more information I have given about my kids up front the more prepared caregivers and teachers have been. They watch for the overstimulation or try and redirect them when they start to disengage from tasks rather than punitive measures for disobeying. These kids can't be treated differently but they can't be put into situations where they are set up to fail either.
Good luck to you and your child, I wish you all the best and hope your kindergarten experience turns out as successfully as mine. Keep advocating!!!