What are the best lenses for starting out with a Canon T2i while still covering a whole bunch of creative techniques / teaching photography (and also theory) to beginners
Assuming that your T2i has some lens, that is the best one to start. Learn how to use the camera. Once you have taken a few thousand photos, you will know what kinds of shots you like, what your needs are, and then it will be obvious to you what types of lenses you should get as a second, or third lens.
The "kit lens" on the T2i was the 18-55. Its a cheap lens, but will do the job. Many folks like the Canon EF 50mm F1.8 as a second lens, its a much faster prime. But if you like to take sports or wildlife photos, you will want a longer telephoto zoom.
Don't hang around photography forums, go shoot photos.
And get a decent post processing software package, such as Aperture, Lightroom or Photoshop Elements. Even beginners can do amazing stuff with these packages, an they are all under $200.
Edited: per @mike, the 50mm is EF mount, works on full frame too.
The one that comes standard with the camera...like 18-55? or 24-70?...is a great lens to get your feet wet. I've found that it's got enough of a range from wide to telephoto to satisfy most people's interests. Once you've learned what that lens can do, you can think about getting a telephoto or something even wider.
I think the BIGGEST thing, though, is not the lens, but the ability to override the automatic settings. If the camera does not allow manual adjustments of aperture/shutter speed/ISO, you're better off picking up some old manual lenses on eBay and FORCING the students to do it all manually.
Understanding how the various controls affect the image...whether it's exposure or depth of field or motion blur or what have you...is really what's most valuable to creative expression. Allowing for mistakes because you can't automatically get perfection...understanding how to REPRODUCE those mistakes at will and finding a place for those techniques in a creative repertoire is really what it's all about.
As long as the glass is clear and clean, you're set.
Another thing that I've worked with for workshops is the LensBaby - http://www.bhphotovideo.com/c/product/580433-REG/Lensbaby_LBM2C_Muse_Special_Effects_SLR.html - all manual, aperture is controlled by dropping in a matte black cardboard disc with a hole punched in it...you get about 4 or 5 sizes...and it gives you tilt/shift capabilities too.
UPDATE: and now, understanding that you're doing self teaching instead of doing classroom/workshop work, echoing @PatFarrell, go out and shoot photos. Worry about composition and light before you get more glass. You've got plenty of range with what you have, and as you shoot and discover your style, you'll know what lenses to pick up next. I bought my first SLR in 1983 with a 50mm lens. I got a 35mm lens in 1991. When I went digital, I used the the 24-120mm that came with the camera for about 6 years before buying a SWEET 14mm lens. I've only purchased a telephoto (600mm) in the past year because I needed it for a client project.
You'll figure out the next lens to buy as you gain more experience.
The lenses you buy should be driven by the pictures you're trying to take. When you're beginning, you may have a general idea of what you want to shoot, but the early days are for experimenting and figuring out what types of photos make your toes tingle.
The recommendations for starting with what you have are spot on. Don't worry about more lenses until you run into a situation where what you have won't give you the image you're trying to get, then figure out what kind of lens will help you get that shot (by asking around this place, for instance). The answer will be a lot different if you're doing portraits than if you're doing street than if you're doing wildlife than if you're doing landscape than if you're doing macro. So don't make equipment purchases until you know what direction your work is headed. (invariably, someone always says "buy a 50mm!"; in fact, that's not necessarily the right answer. see here: Do I really need a fast 50mm lens? )
Also, before you buy, RENT. you can try out a lot of options by renting lenses from a place like borrowlenses.com or lensrentals.com, and see if that lens both solves your problem and is the right quality.
But most importantly, focus on the image, not the gear. The gear is there to make the image possible (or easier). It's fun buying gear (we all know that), but unless you want boxes of stuff you rarely or never use, teach yourself to NOT BUY stuff until you have a specific need for it and you can explain how what you plan on buying will solve a problem between you and the images you're trying to take.