I never heard "ikaga desu ka" in any of my Japanese classes, either.
But, I've heard a Japanese acquaintance use it. Also, I've heard a Japanese seiyuu use it, as well as an employee at a Japanese grocery store.
Btw here's an online dictionary
http://www.csse.monash.edu.au/~jwb/cgi-bin/wwwjdic.cgi?1C"Genki," a "-na adjective," means like... "health(y); robust; vigor; vigour; energy; vitality; vim; stamina; spirit; courage; pep;" according to that online dictionary. If someone asks you "Genki desu ka" you can reply with a "hai" or an "iie." You can say something like "Hai, genki desu" or "Iie, genki ja arimasen" or "Iie, genki ja nai desu." I guess your teacher kinda simplified the translation a bit...
But "ikaga desu ka" is not a yes or no question, fyi.
Konbanwa and konnichiwa end with the "ha" symbol because they're some of Japanese's odd quirks. It's similar (or exactly the same as) "Watashi wa Rampage desu" using the "ha" symbol, but being pronounced "wa."