I teach 7th grade, and I had to tell my students at about 7 weeks, because my m/s was so bad, I was so nauseated and sick all the time, that it was obvious! One day we switched a student from another class into mine, and when I gave him an extra desk, it turned out someone had left an old apple in it and it was rotten and smelly. When the kid told me, and I went over to it, I almost lost it, and I had to admit to the kids that I was really nauseated, and asked if someone could clean it out (one of the boys stepped up!). After that, they all started asking me how I was feeling, what was wrong, mentioning that I didn't seem like myself, etc. I could tell they were speculating, so I just went ahead and told them. They were really sweet about it, and they've been really great about giving me a break when I'm not feeling well.
This group was really immature when the year started, and we've worked a lot on self control and monitoring our own behavior. I noticed that once I started getting sick, they really stepped up their maturity with monitoring themselves and each other to manage their behavior. They are SO much better now than at the beginning of the year!