I'm also a teacher currently working on integrating my Ipad full time in my school, but I kinda did it at the last 3 months of last year so I can only talk for the few apps I used.
AttendanceIq: Great and fast attendance taking app where you can make notes and add points of view to what is happening in the classroom, I edited a few fields and added the talking too much, working very hard, great presentation etc.
Istudiespro: this is a great app for keeping a record of what classes and what time and a general calendar I used this app in combination with my univermsity classes I attend as a student too.
Note plus: not used in class still but I have been tryhing it the last few days and is a great note taking app, both hand and type written.
Instapaper: for downloading web articles or pages to be later viewed offline if necessary or just to add reminders of things I need to read about.
Presentation timer: a simple app I use in drills or wherever I need a timer for my students.
Grammar up: a good source of quizess.
Groovy grader: a simple app to grade my tests very quickly, if there are 30 questions it gives me the grade for lets say a test with 27 correct responses, this app is just cause I'm lazy I know
Grade pad: this is an iphone app, but is a great way to develop rubrics and get quick easy and mistake free grades for various subjects.
I'm currently playing around with Teachers attaché and teacher's helper I'll make sure I post whatever I think they work for.
Hope this helps.