Hi citygirl, welcome to the forum :)
Since I have started Humira I actually get sick far less often - most likely because I am now extra careful to avoid germs and sick people. The precautions I take are to avoid being around sick friends and family whenever possible, and I wash my hands religiously. It's just a habit now - whenever I arrive at work, or back at home, the first thing I do is go wash my hands. I'm also a lot better at not touching my eyes nose and mouth with my hands.
As a teacher you'll probably have more trouble with avoiding sick people, but frequent hand washing and avoiding touching your face with your hands should help! I have also found that when I do get sick, it's actually much milder than normal but it lasts forever. Colds don't hit me as hard as they used to, but I'll have symptoms for a month or so.
I hope the Humira helps with your Crohn's! It worked very well for me when I started on it.